I'm thinking the same as Habu. You might consider delivery time as a factor in your choice. If the Colerain is availble now, it wouldn't be a bad choice, you're just going to put a lot of work into the outside finish. Joe at Oregon barrels says he can get a barrel to you in about three weeks. I haven't used one of his barrels yet, but I hear good things, like equal to Rice. I plan to order from him on my next build. Rice is fantastic, but you may be waiting weeks or months for your barrel.
I would definitely choose the .54 with that heavy tapered barrel. I am currently building a Hawken using a .58 Colerain barrrel, tapered 1-1/8 x 1 x 33". Trying to keep that weight down and keeping the length shorter for field use. Good luck. Bill