I have lots of experience with both. I love the warmtone RC paper. I use the pearl finish. It's a very nice paper and tones wonderfully. I use it for alot of work 8x10 or smaller. The warmtone FB version is an absolutely beautiful paper. Much different feel than the RC physically speaking. I honestly feel the warmtone RC offers greater flexibility with thio sepia toning, but the FB version is more flexible in selenium. Wonderful split selenium toning can be done on the FB version. Both are great papers and I will continue to use both. I think larger prints, 11x14 and up look better on a FB paper. Smaller prints, 8x10 and 5x7 are nice and quick on RC paper.
As for flattening FB prints, I have a bunch of acid free matboards about 16x20 inches that I will inter-leaf the prints after they dry and stack heavy books on top of the stack. Give it a few days or a week and the prints will be flat.