Whatever brand/color you buy, its the batteries that'll kill your wallet...to get enough batteries to keep you going without having to wait for the charger. I've always had Yellow tools, 18VXRP stuff which was my entry into the cordless world. I used them for 2 years installing garage doors in new construction homes, and retro-fitting new doors into old homes. They held up great, heavy tools, but they did the job very well...Ryobi stuff fell apart after a few months of rough abuse being dropped off ladders and such. I dropped my 1/4" impact several times off the ladder onto the concrete floors on the jobsite, never had any problems. Finally gave my 1/4" impact, reciprocating saw, circular saw, and hammer drill to a friend after all the batteries of mine had died, he still had good batteries so I just handed my old stuff over to him to keep using. I had purchased a kit with 2 adapters and a couple batteries to go from the 18V to 20V stuff. The only thing the adapter didn't work in was the cordless vacuums, well they worked but on the one you couldn't close the battery door. I have a 2nd 18V reciprocating saw and I've been using the 18V-20V adapter on it for quite a while, the adapter just stays in the tool. To swap adapters its a pain, many of them get quit stuck in the battery slot and make it hard to remove. Pretty much all my larger tools are now 20V, I do have some 12V and a couple of 7.2V tools though.
Honestly its really going to come down to what color you like, they all really perform the same in the real world. Pick a platform and stick with it, batteries and chargers are the expensive stuff, once you get several then just buy bare tools and save yourself. I have a few 4ah and 5ah battery packs, but mostly I just use the little 3ah packs unless I'm running the 7-1/4" circular saw, grinder, or other high demand tools.