Which Lapras to power up?

FB/IB will do more damage than FB/DP. Forget this retarded IV requirement bullshit. Movesets are far more important, 7 base stat points have no noticable impact in battle.

IVs make little difference in battle because damage is rounded, so the difference in your attack and defence IVs are not going to change the damage you take from quick moves, which are the majority of damage. They affect charge moves but only slightly. Like 2-5 damage depending on the base damage of the move, which isn't much when you consider how many times you'll actually use (or get hit by) a charge move.

Ice Beam is worse than Blizzard, but Blizzard is one of the strongest charge moves in the game, so being weaker than it doesn't make it bad. Ice Beam is on par with your typical 55/65 damage 50 energy moves.

If you truly only power up Pokemon with 37/45 IVs or more, you've answered your own question - don't power up either of them. The first one has a suboptimal moveset that is not worth investing in as a competitive player (which you are if you care about IVs) and the second one doesn't have good enough IVs for you.

If you can forget this misguided notion that IVs make a significant difference in battle, then power up the second one.

Lapras are pretty rare and if you get one with a good moveset and pretty good IVs, you should power it up.

Since you want an attacker, FB / IB is an excellent move. The main difference with Blizzard is that it is a 50% charge move vs .a 100% charge move. Blizzard is more of a "big lethal blow" move whereas IB is a continuous spam charge move. Both are very good for attacking and depend a bit on your style of attacking (e.g., for Snorlax, do you prefer attacking with Hyper Beam or with Body Slam?).

Frost Breath is also awesome as a spam quick attack, especially against Dragonite. All in all, I say go for it!

Like the above fellow just said, 7 points of IV will not make much difference, the Lapras with IB will do much more damage than the one with DP even with less IV. IV are important? yes they are, but moveset is even more important.

The true question is, how rare lapras is for you? you only find these in the whole game or find others?

If i've found this second lapras i would definately power it up because the moveset are good and the IV too because i never even saw one, but some players have dozens....
If you know you can find another one with realtively ease, wait for one with blizzard, if you're like me, power the second one.

hope it helps.

Another thing to try is just attack 3000CP Dragonites in gyms with both and see how you do.

If you have limited dust and candies, you can always use both Lapras as attackers now and if they work out ok for you, leave them at Lvl 20.

Since candies to max one to 30+ and higher is very expensive and you can still easily dispatch any Dragonite already, you can just wait and save all those candies for you ideal mon.

At lvl 20, it's already 72% as powerful as it can ever get at 40 so if they aren't ideal for you, you can still use them in your team, while you wait.

Lapras#2 can serve both as an attacker AND a decent defending Lapras. Ice beam is faster and hence harder to dodge than Blizzard. Also, Gym AI uses Ice beam more frequently and thus minimizes energy waste. And trust me, Lapras is better as a defender.
For the #1, however, it is a 90iv+ Lapras, which many players (I myself included) still don't have. The way I see it, you are better off to either:
i)Save it for future trading; or
ii) Max it out and put it in gyms, which makes you really proud (at least for me).

And if you really really cannot decide, why not both? Why not walk with your Lapras? After all they are all your babies!

Lapras doesn't really have a bad moveset (Dragon Pulse is till a strong charge move). Ultimately I'd try and power both up, but no. 2 would be my priority.