There are many guns that are currently on the market for spraying foam. There are three main guns for a reason. These three main guns are the Probler P2, the Fusion CS and the Fusion AP. Each has it's own advantages and disadvantages. 

I will give a quick explanation on each gun, so you can find out which one is best suited for you. 

Probler P2

The Probler P2 gun is often referred to as a P2. This gun was created by a company called Glas-Craft. This company no longer exists. It was purchased by Graco back in 2008. Because of this, every year Graco increases the pricing in this gun, making the parts more and more expensive. Usually companies will supply parts for customers like this for 10 years after a buyout. Hopefully this stays a popular enough gun that Graco doesn't discontinue it in 3 years. This is a major reason for a lot of people switching over. 

This gun has a really nice spray pattern, which you can slightly adjust (using a nut driver) so you can have some control over it. 

The way the material enters this gun is from over the top. Each line is connected to a simple ball valve, which is attached to a "removable block" which contains the side seal. Each side has its own separate block is easily removed (by removing two screws) so you have easy access to inspect the side seals, as well as the chamber's sides. 

The chamber itself is made up of two separate pieces. It contains the mixing chamber, as well as an insert. This allows you to quickly change from a normal pattern, to a wide pattern (unlike the next to guns)

This gun needs multiple tools to disassemble and put back together. 

P2 Conclusion:

Because of the easy side seal removal, spray pattern adjustment, as well as two part chamber, it makes this a very nice gun. The down side to this gun is It has a lot of parts, which seem to be getting more expensive by the day. Making it a very costly gun to operate, as well as a little harder to rebuild (but still not to bad). Also there is no way to test the gun to make sure all the seals will work properly after its rebuilt. If there was something you missed (such as scratches or a damaged/pinched oring, you will be rebuilding it all over again. 

Fusion CS 

The fusion CS gun is a unique gun. It was designed to have a "special" fluid squirt out through the chamber every time that gun is triggered. This fluid keeps the gun from plugging up and crossing over, if used correctly it really helps on how often the chamber needs to be drilled out. This gun was also the first truly adjustable gun that was released to the market. At the back of the gun there is a dial you can adjust very easily (all you need is your hand). This simulates different chamber sizes while spraying, all from using the same chamber. You can either spray wide open on large surfaces, or dial it down to do some more detailed work. All with the turn of a knob. 

Finally the major advantage to this gun is the light weight. It is much smaller in design and about lighter then your average gun (by feel anyways. Graco's manual says its .1lbs heavier than the AP gun) which makes a huge difference after a day of spraying. As far as parts cost for this gun, it's middle of the road. Still a little expensive, but if everything works the way it should, there should be little maintenance on it. Also if there is any issues with the front housing, it's easily taken off and an new one out on. This will get you up and spraying again in left then 5 mins. All the tools you need to rebuild this gun is a 5/16 nut driver.

The down side to this gun is really one of its biggest advantages. The special clear shot fluid. When this is working properly it works great. When it's not, it can be a nightmare. As a sprayer you really need to pay attention to patterns, as well as making sure you have clear shot coming out after every trigger. If you don't, the chamber will plug up a lot and eventually cause the gun to cross over, creating a lot of downtime. 

The chambers in this gun are just one piece. This  makes the complete chamber cheaper to purchase, but causes you to pull the front end apart every time you want to change from a normal pattern, to a wide pattern. 

Like the P2 gun this gun has a lot of parts to it which adds the the rebuild time. Also like the P2 there isn't any good way to check the gun to verify that all the seals will hold after a rebuild.

One of the ways this gun was made "lighter" is because of plastic parts, as well as smaller parts. This makes this a very delicate gun. You can not drag it around like you can with the other two. It's best to carry around, so it doesn't break on you. 

CS Conclusion:

This is a good gun, with some great options but is only for experienced sprayers. Handing this to someone new while make them frustrated, and hate the gun. If you are a new owner operator, I'd shy away from this gun. I've seen this gun put guys out of business because of the issues with it (of course there were other things, but issues with this gun really hurt their reputation). 

Fusion AP 

The Fusion AP gun has been around longer then the other two for a reason. It's very easy to work with. It's the best gun to put in the hands of a new sprayer. Like the CS gun, the AP gun has a quick and easy to remove front housing, limiting downtime from a front end issue. This gun also has a one piece chamber, causing you to swap it out ever time when you want to change spray widths. 

Out of all of these guns this is actually the cheapest. It's cheapest to purchase as a whole, and it's the cheapest for spare parts. If you look at how many parts there are to make this gun operate, you will notice it has the least amount of parts. In a perfect world all you need is a nut driver, just so you can unscrew the coupling block from the gun. Once it's off you should not need any tools (but it's a good idea to have a small flat screw driver, and a small adjustable wrench on hand)

The disadvantage to this gun is its weight. It's the heaviest gun of the three. Some people find it bulky. 

AP Conclusion:

This is the most popular gun on the market for a reason. It's the easiest to use, it's the easiest to rebuild, and surprisingly it's the cheapest. This is a strong gun which can take being dragged across the floor (but still not recommended). You can put it in the hands of a pro or a rookie, and they can both easily use it. Downtime can be only mins, as long as you have a spare complete front end. One of the best parts about this gun, is being able to check it to make sure the seals are all working good, before putting it on. This eliminates the potential for a lot of head aches, as well as downtime. 

Over all Conclusion:

1) Taking everything into account (as well as personal experience), I would rank the Fusion AP as the best gun on the market. It's cheap and easy to use. 

2) Runner up is the Probler P2. It can have a very nice spray pattern, the adjustment, and two part chambers are nice options for it. But sadly it's the cost of this gun which hurt it the most. It's not a Graco gun, because of this, they will soon be making it obsolete. They are trying to make everyone switch guns by pricing it out of the market. If you really want to use it, it will cost you.  

3) Lastly is the Fusion CS gun it has great ideas, and features, but is almost to fancy for real life. I wish I could have ranked this gun higher, but It needs a lot of attention to detail while using it. If you have one and are making it work good for you, congratulations you are one of the few! Enjoy it and keep doing what you do.