It really comes down to your playstyle. 71 gives you more survivability and sustained damage, while 74 gives you more burst, but less survivability. Either playstyle works fine, so pick your favorite.
Edit: going from 71 to 72 causes approximately a 7% drop in your secondary stats, which (for a fully geared player) comes out to about the equivalent of losing four sockets in your gear. Getting the engineering helm at 72 typically adds 2.5 sockets' worth of stats over the PvP helms, so while the engineering helm is nice, it's not a big reason to go to 72.