I have the Ryobi brad nailer. It's awesome. Biggest difference is that whereas the Milwaukee has 0 delay, the Ryobi has very very short delay between trigger press and firing. It's never bothered me. In the Roybi's favor, it never overheats and will go all day ... the Milwaukees have overheating problems. Also Ryobi batteries cost less than 1/2 as much if you get them on sale. On this particular tool, given the Ryobi and Milwaukee at the same cost, I'd probably get the Ryobi.
I've recently been looking into this as well. Milwaukee's did not hit it out of the park with their M18 nailers as they did with a lot of their other cordless tools.
If you are looking at the 18ga brad nailers, I do think the Ryobi is the way to go. Smaller, much less expensive and works just as well but if we are talking 15/16ga finish nailers, they are much closer performance wise with a slight advantage to the Milwaukee as they have no wait time after pulling the trigger, The big difference is the price but if you can get the Milwaukee 15ga kit for $199, i'd probably get that one.
As for 15 vs 16 ga... That's a big debate with no "right" answer. Mostly depends on what you want to use it for and what is easier to find locally nail wise? Hanging doors and such, I would get the 15ga. If you just want it for trim, the 16ga is better as it leaves a smaller hole but the 15ga has more holding power. I've been using a 15ga for years and have been happy with the results.
I may pick up the Ryobi brad nailer but will wait for Milwaukee to come out with the Gen2 versions of the nailers before buying more. I'm hoping the Gen 2 will be majorly improved and they they come with up a cordless pin nailer too.
