Tampa won't be taking a goalie at 15. They have 4 good goalie prospects, 3 of which are under contract and will be in North America playing in the AHL/ECHL this year. Karri Ramo is quoted as being a "stud with starter potential" and will be playing in the AHL this season. Vasily Koshechkin was the 2nd best goalie in the Russian league this year with 10 shutouts and insane numbers, and he's only 21 or 22 years old. Jonathan Boutin and Gerald Coleman already have a year of pro experience and will still be around this year...theres no reason to use their 15th overall pick (if they don't trade it) on a goalie. My money is on an offensive D-man or one of the better forwards who slips into the 15 range.
But I guess to answer your question as to who Tampa would choose between Bernier and Helenius, it'd almost definitely be Helenius. Tampa has a history of, or even a fetish, for drafting/signing BIG goalies. Helenius fits that mold at 6'3, Bernier doesn't at 5'11.