Double plastic boots are the standard of glacier mountaineering and most use them. Hoever, I've climbed Rainier (and most of our Washington volcanoes on many occasions) in leather boots. Lateley, I've switched to La Sportiva Trango S GTX. Last summer, I led a climb for three days on Rainier (same route as yours) in these boots and had wonderful luck. Without the warmer liner, you'll definitely want a roomy boot with a warm sock (I prefer Darn Tough or Smartwool). I also carried a vapor barrier sock but never needed it.
As far as climbing, I find these boots to be great for moderate front pointing and all French technique. They don't accept step-in crampons but are great with hybrids (and offer a bit of extra flexibility which makes them far superior on the long approach to Camp Sherman).
Bottom line- Ryan and I use the same boots and both have had great experiences on Rainier.
A few good places to look in Seattle (if that's where you are): Second Ascent, REI, and Pro Mountain Sports (the last being my favorite). I also tried on Kaylands which were similar but didn't fit me quite as well.
Have a great climb!