I have had nothing but problems with my two Stanley roofing nailers purchased several years ago. They drive nails crooked, and many nails need to be driven home with a hammer. I have heard other pros agree on easy rebuilds with a senco. If I am buying a tool that is new to me,and have no previous experience comparing other brands of tools that others have used at a jobsite,I will always go with Porter Cable. Every Porter Cable tool that I have ever bought [including two air nailers] is the first tool that I want to use ,because it has the features that you want to do good skilled work and is light enough that it does not punish the operator. I have some functioning tools that gather dust,just because they do not feel right in my hand. There are no Porter Cable tools among this group. The feature I look for on an air nailer is the ease of loadind and an adjustable depth of drive. Also try to determine how easy it would be to accurately place a fastener with the positioning aid on the nail gun.
Not all brands of tools have a good product line across the board. If you get a Milwaukee drill you really like,that is no guarantee that you will like Milwaukee circular saw. Your best bet is to try a tool before you buy it. Some stores have a 30 day buyback if you do not like the tool.
Milwaukee used to be the most bombproof of all motors. I bought a used metal body Sawzall for 20 bucks,and I have had it so hot making cuts in oak that you needed gloves to hold it. My two year old Super Sawzall puked a cheaply made trigger switch before the armature took a crap and turned the thing into a paperweight. When the quality toolmakers started selling their stuff at Sears,Lowes,and your local hardware store,the quality just is not as it used to be.
Sorry for the slightly off topic rant, but the advice on not always sticking to one brand of tools is something to consider. That is why Dodge has the Cummins.