Leif,
If you don't have a 15# rig, I'd bring that rather than 12. If you've got another 15, bring a 12 also!!! 15 and 20 were the most used on my trip, but I went up to 25 for the bait. That belt is the Braid Brute Buster. If the fish is right in front of you doing circles, you can really put the hurt to em, but when you run down the rail it tends to move out of place so sometimes you have to re-center it on your waist. I'd highly suggest it for your trip, along with maybe the marlin harness. I got that too, but didn't use it (the big BFT didn't show up...). Anyway, if you've got to choose between 12 and 15, go 15. You'll get sawed off/broken off too much on 12 for it to be worth it, IMO. Oh yeah, and on the heavier line, like 30, 40 etc. (actually all line...) fish P Line. I only had my 40 and 20 set ups with P Line but wish I had P Line on all my reels. It's not only super strong, but also really thin so it tends to get bit better. I was not impressed with the floroclear, especially the knot strength, but I'd like to give it a few more chances.
-Brian