Definitely go for a "dry" type immersion suit, so the goretex would be my choice. HOWEVER, what you wear underneath it is vitally important. The suit will protect you from the initial cold shock as you enter the water but it will not provide you with insulation. If you do not therefore get into a raft you will still die relatively quickly from hypothermia, you've got about an hour in 5 degree water unless you are properly insulated. Several thermal layers worn beneath your immersion suit would be just the job.
Concur with the comment about a HEEDs unit, or STASS as it is also known, worth their weight in gold and have saved the lives of several helicopter aircrew in ditchings.