which Lapras to power up?

Lucky! I prefer ice beam. Blizzard doesnt normally do enough to KO Dragonite (maybe at a higher level) and leaves me with the decision of wasting another blizzard, or finishing it off with frost breath to save the blizzard for the next pokemon. I dont have this problem with ice beam.

Power up the second one. Keep first one for prestiging. But seriously, those Lapras can already have take CP 3000+ with dodging because I think Lapras has high hp.

Because the odds of facing back to back Dragonites in a high level gym are pretty damn good, the Ice Beamer would be my choice. Either move will be killer, but if you're dodging, Ice Beam allows you to recover your rhythm quickly and often sets you up for a charge shot to start the next Dragonite.

Ice Beam all day long for me.

FB/ IB better !!!

If you set aside all the stupid undesirable water and mixed water movesets, F/B deals the most damage with F/ib a clise second. Ice Beam is a bit more flexible though.

I think I often need three Ice Beams to finish a Dragonite, which would translate to 1 Blizzard and a second Blizzard with overkill wasting some damage.

If you attack enemy gyms with other players, multi-bar charge moves are even more important and beneficial for not wasting energy.

All that being said though, Gyarados with B/O can also be a good attacker into Dragonite.

If it were me I'd keep both of them at a lower level for prestiging. Lapras and other Ice types are great for prestiging Dragonite, but I favor other alternatives for attacking. That said, if you have unlimited dust and candy and want a Lapras attacker, then the better option depends on your dodge/attack strategy and the specific defender timings. I believe IB performs better in more cases than not, but there are many exceptions.