I would focus on the X5 set (Brushless) and ignore the X4 set; the improvements to the tools overall is worth the modest cost bump, unless you can get the X4 as a huge discount.
I recently started replacing my old Craftsman 19.2V with Milwaukee M12 gear. At work we use Rigid 18V stuff, and just recently picked up a big X5 set. It is all really nice gear, and I have no issues recommending it.
Honestly, if you are just a DIY'r, of any stripe, The Ryobi 18V stuff makes for pretty good deals, and the variety of tools they offer is insane. Several YouTubers I follow use a variety of Ryobi's cordless stuff and rave about the value. There are times when I really wish I would have went that route, but (as far as I know) the Milwaukee is the only 12V line that has a drill w/a 1/2" chuck and hammer drill capability. I haven't regretted that purchase at all; the smaller, lighter tools are easier to store, easier to use, and I have never had an issues with lack of power or runtime out of them (for my purposes, YMMV).