If I were starting from nothing, I'd probably buy Dewalt cordless tools. A buddy of mine is all-Dealt, and I've used his stuff enough to really like it. That said, I started out with Ryobi because, like a lot of guys, I got a combo package of drill & impact driver for Father's Day or something years ago. Since I have One+ batteries on hand and don't want to replace it all, that's what I stick with. There's absolutely nothing wrong with he Ryobi stuff. It's tough, powerful, works well, and not a single Ryobi tool I own (2 drills, driver, tire inflator, 18 ga brad nailer) has ever broken. I've had to replace a couple of batteries, but that's a constant no matter what you buy.
So why would I go Dewalt if I were starting over? I just like the compact size of their stuff better is all. They work exactly the same as what I have, to be honest. And if it comes with a power cord, it's probably going to be Dewalt. My chop saw is Dewalt, as is my 1/2" drill. The table saw I'm going to buy will be. If I buy a planer it will be Dewalt too.