In the past decade, the following has happened:
Dewalt changed tool lineups from 18v to 20v. They just recently supplied an adapter to use 20v batteries in 18v tools, but that's a new development. For the longest time, the only way to get Lithium-Ion was to upgrade to the 20v Max line (though, they did finally release Lithium-Ion batteries into the XRP line).
Milwaukee changed up from V18 to M18 - batteries between the two are incompatible without modification.
Pretty sure Makita changed up their 18v batteries from a stem-style (1822/1833 batteries) to a slide style when they went from NiCad to Lithium-Ion.
Sears/Craftsman introduced a 20v line of tools alongside their 19.2v lineup - batteries were incompatible. Ended up finally cancelling their 20v line, and introducing Lithium-Ion for the 19.2v line.
Ryobi, in the mean time, has stayed consistent, not changing up battery packaging, and not forcing you to start buying a new tool line with that new battery style when they introduced Lithium-Ion.
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Whatever choice you make, you'll probably be happy with the tools.
The vast majority of users don't need that extra 10 foot/lbs on their impact wrench, or the extra 20 inch/lbs on the impact driver.
The vast majority of users of users don't need and won't notice many (if any) differences between brushed and brushless motors.
The vast majority of users won't be in a position to take advantage of a slightly more compact tool.
Ultimately, if you buy Milwaukee/Dewalt/Makita, you're going to be happy.
I personally believe you'll be just as happy with Ryobi.
I'd only steer you away from Craftsman C3 (for reasons explained above) and from Porter Cable (due to lack of breadth in offering).