I haven't been on either boat, but did a bunch of reading/research earlier this year prior to booking our trip on the Odyssey.
One big factor for us is that the Thorfinn basically is anchored and doesn't move. All diving is done from little boats which take you to the wrecks. This is both good and bad, the good being that there are fewer divers on the wrecks and that groups can be divided more by ability. Bad being the boat rides and most importantly for us, that dives are pretty much "scheduled". We prefer to dive from the mothership on a liveaboard, giving us the dive freedom to dive as much or as little as we want, and when we want.
The reviews of the Odyssey are consistently better than the Thorfinn, and basically it just seemed to be a better match for us. It does seem that if you're a serious technical diver though that the Thorfinn gets the nod from that crowd.
Undercurrent has quite a few reviews of both in their chapbooks and on their website, so reading those is a great idea to help you make up your mind. As a caution, be aware that often Odyssey has to be booked 2 years in advance. We've already booked our August, 2008 trip with them as they only had 2 weeks available in the summer when we could go.