Depends what you're after. NMG offers more protection when stacked with nanoweave and is a reactive shield, I'd suggest this if you're looking for a quick health boost if you get jumped. Resist requires you to have it active before you start taking damage, and offers a total of less health than the NMG/Nano combo, but its much more reliable, always offering the same protection no matter what the charge is at and recharging fast enough to be up more often than its down. I'd suggest this if you're the one starting fights, or if you find NMG's recharge too slow.
In most cases? Nanite Mesh Generator (Or Adrenaline if you kill plenty of people) since you can pop it when you need it and ignore it when you don't. It's much better for the moments where you suddenly find yourself taking fire and it's still quite useful when you pop it it anticipation of a fight. Resist only really shines if you have it ready to go before every fight which requires a decent amount of prediction.
De mesh generator for our zerged lives, man! You take far more damage, can activate when that pesky Medic appears in your face, and not worry about your health bar! As for resist shield, don't waste your time, better protection is better than more frequent protection.
Situational? If you're running with a squad and have medics to back you up, I'd run resist. The other alternatives leave you waiting on regeneration while resist has much more uptime. Adrenaline/NMG for soloing for sure though.
