UOSteam I think is a bit newer, but both are unsupported by their devs at this point as far as I know.
UOSteam has a bit more powerful macro interface. But on the other hand it cannot handle the old-style gumps that we have here in the macros. Razor also has a bit better dressing mechanics as it removes conflicting items before trying to equip the new ones. Other small stuff like that.
Some ppl have a white flash flicker in their client when running Razor, maybe from using compatibility mode, which doesn’t seem to happen in UOSteam.
I personally have both installed for different situations, but plan on using UOSteam when not afk macroing.
I do get flashes when using Razor. Setting the compatibility option will remove those flashes. I’m not sure how Windows handles these things but i’m guessing it has something to do with the visual effects that are absent in Windows XP.