I ran the numbers. This is only the base gear though (whatever runesdatabase.com shows). I'll have to get in-game and play around with Item Preview to include plussing, dura, and tier.
With full lvl70 4-peice set:
Crit Rate: 800
Dex: 656
Dmg: 260
PA: 4020
Str: 224
With 3-piece lvl70 and 1 lvl65 (either helm or shoulders):
Crit Rate: 570
Dex: 456
Dmg: 200
PA: 5040
Str: 74
So, yah, I guess the full set would be a bit better. Since once you add the PA from the extra dex and str (plus hero pot) the total raw PA increase probably isn't worth the loss in dmg and crit rate.
I would try to update your chest and boots first though, if given the option (probably won't since those are probably harder like GC sets), since those give PA, and until you have 3 pieces of the new set you could probably use the extra PA.
Edit: And according to runesdatabase.com the +12 bonus for both sets, for the helm and shoulders, is identical (kinda lame, seems like it should have gone up). Do keep in mind though, set bonus doesn't go up with tier, so if you go really high tier it might then be better to break the set.