So here's the deal: One, either a down or synthetic bag will be fine. Get one rated to between ten and 20 degrees. And you should be OK with open-air bivys. I guess I'd recommend a light bivy bag, whether using a down or synthetic bag, just because the extra shelter could come in handy.
Two, check the weather forecast, and don't be shy about packing a tent if it's marginal. Rainier is a big, Alaskan-style mountain, one that creates its own weather. And while my bet is that conditions will be fine - clear and sunny, with high winds above 12,000 feet the worst problembig storms can hit.
As for synthetic-bag durability, it's true that short-staple fibers such as Primaloft supposedly do not last as long as long-staple fibers such as Polarguard. But in reality, good manufacturing goes a long way toward overcoming this problem. Integral Designs bags are fine. A good one for your trip would be the North Twin ($220), a really nice, ten-degree bag.