Which Reinhardt should I use?

They're pretty much the same in performance so I wouldn't worry about it, just merge into either one.

+ res is probably better, even if it's small there's more of a chance of surviving a magic attack, especially if he has cavalry buffs

Reinhardt would prefer an +Attk IV in general, but I would say go neutral since he doesn't really benefit from having an bolstered defenses or resistances in general (unless you're worried about dealing with Ridersbane or Zanbato, but he has atrocious speed in general and won't really survive since he WILL be doubled.)

He defense is 30 with +DEF. Not bad, but his speed will still get him in trouble

Neutral.
-def hurt his already middling defense, while neutral Rein (with or without lance breaker) could be just fine again most lance and sword unit.
well, Rein isn't that hard to search since he's available on 4*, so you can build the neutral while waiting for the +atk to arrive