It comes down to personal preference. Which one has a playstyle you like more. My suggestion is to buy a set of Rare armor for each and give each a try for a while. If you decide you hate one set or the other, use a Master (or Black Lion) Salvage Kit on the armor you don’t want. You should average an Ecto about 90% of the time which can really help recoup your losses (if you bought the sets via buy orders, you can even MAKE a bit of money this way sometimes). Then you can start working on an exotic set for whichever build you end up liking.
And I’d actually disagree with the previous two posters about which is best where.
Condi Necros have a few major advantages in zergs. Epidemic is ungodly powerful in a zerg fight if you pick the right target. The Staff is probably our strongest zerg weapon and it’s strongest on condi Necros. And since condi specs are unlikely to be focused in a zerg fight if they position correctly, they will be able to toss out their damage in the manner which they prefer: more carefully and deliberately, building their damage. It’s true that you run into condition caps and AoE condi clears more easily in zergs, but it is profoundly unlikely that everyone on the enemy team will constantly be capped for every condition and having more condi-spec’ed characters makes it easier to overwhelm the enemy zerg’s condi removal.
Power Necros can still be useful in zergs but they have way less at-range AoE damage than Condi Necros. They can still use the staff deftly, but their damage will be somewhat stunted compared to their true heights. Conversely, I generally find my necro’s Power build letting her tear through enemy roamers in 1v1 and even some 1v2 situations (it’s very difficult for Necros of any stripe to fight more than 2 opponents at a time, and even fighting 2 is extremely dangerous if they’re any good), whereas my Condi build has a rougher time dealing with targets who can disengage well.
I main Ele and Necro, though I have an alt of each profession at level 80.
How to Condi Reaper on a budget
Everything I say is only in reference to PvE and WvW.
