I was wondering if anyone could comment on which one is more water
resistant (personal experiences welcome, but scientific information
would be ideal)

Oil-modified urethane
OR
Catalyzed water based urethane (ie: like Traffic by Bona)

I'm going to have my floors redone and since I'm planning on getting
fish tanks, I'd like to get which ever finish is considered most water
resistant. Obvioulsy none are water proof, but which finish would
stand up best to a small spill that might have gotten missed and sat on
the floor for a day or two until it evaporated.

Thanks,
Harry