I've found plastic boots nice for long periods on the front points. They don't suck. The Scarpa Inverno is surprisingly comfortable to walk in and it is bombproof.
As for Rainier, I don't think it matters much. There are some comfy plastic boots out there for that, there are great leather boots, and each have seen thousands of happy ascents. I do believe that plastics are more likely to fit your feet comfortably right out of the shop, while leather/synths need to be bought first and evaluated. Whatever you do don't buy a pair of boots and show up with them in the box at Paradise.
Have fun on Rainier, whatever winds up on your feet.