Which Electabuzz to evolve

The one that is a higher level in my opinion. The difference between those 2 is extremely small. If they are the same level (or very similar) go with the 15 DEF one.

Here's my reasoning: 1 more hp will very rarely make your electivire survive a hit. 1 more def will probably do nothing, but in the very very rare occasion it actually hits a bulkpoint, your Electivire will be noticeably tankier.

Now that I made you read my reasoning, there's another reason (specifically for Electivire) that the perfect DEF is better; Electivire is one of those Pokemon that are effectively "perfect" with 14 HP because of rounding up. So at level 40, your Electivire will be exactly the same as a perfect one.

What w8karpouzi said above, but also note now that HP has been boosted universally across all pokemon and that it is worth less than defense in general. The maxImum gain for 1 HP IV is 1 HP. Also for CP calculation purposes (not that it really matters) the one with higher def will end up with higher CP.

The above says it all already. I'd like to add something: when there's no other reason, always favor the lowest base stat of the pokémon. The lowest base stat of Electivire is def :)