Sizing for undergarments is really important. You might want to bring your drysuit into a few shops, and try on different makes and models, to see what fits you the best. If you can't find anything with a great fit, made-to-measure is an option.
I did this, and ended up buying a 1-piece DUI Duo Therm from Diver Dan's in Santa Clara. My main dive buddy uses a 2-piece Fourth Element, I believe it is the Arctic model. Both are sufficient for our Monterey diving in trilam drysuits, although of course every diver's tolerance to cold is different. His 2-piece setup seems like less of a PITA than my 1-piece, and more comfortable walking around the parking lot with the suit peeled down halfway, so I think I'll go for a 2-piece next time.
Cheers