How good are you at navigating with map and compass/GPS in a whiteout? We just spent an entire week looking for a dude on Adams who'd never been there before either and got lost (and broke his ankle to boot). If you're OK with that, then a late season jaunt up Adams would be the better trip, IMO. You'll have more to look at from the top of Adams than from 2/3s the way up Rainier. The snow is fresh, so above the RTM trail it will be nice (less rocky).
If you think that you might not like that, then there's always a pretty well-beaten path up to Muir. Remember, though, that you can get lost on the Muir Snowfield just as easy as anywhere else if the clouds drop in or a blizzard blows up. Print off the the "Get Your Bearings" cheat sheet (available anytime from the MRNP website) and take it with you, along with the necessary acoutrementi with which to use it.
Oh, and have fun.