Is is better to rebuild them or just buy new ones?

Oriley(parts store) has remanufactured ones for $76 with limited lifetime warranty.

Or should I get the kit and have my mech do the work?

I got a damn leak, it aint the lines.

 

I went with store bought professionally rebuilt ones because I dont trust myself when it comes to doing something as important as rebuilding brake calipers.
I replaced all four of mine with ones from AutoZone, same deal, lifetime warranty, $250 for all four.

 

Unless you can DIY on the rebuild I doubt its worth it. $76 for a lifetime warranty is pretty good. By the time you factor in the rebuild kit(new pistons, seals, and rings) and your mechanics time you'd probably be pretty close to $76 each.
That said, a DIY brake rebuild is pretty darn easy. People always think "oh my God, I can't work on my brakes!!!". If you can manage a rotor change with bearings and seals and lock washers you can definitely manage a caliper rebuild. I prefer to split the caliper halves and use new channel seals, but the channel seals are getting hard to come by. And with British Pacific out of the equation caliper rebuild kits aren't as cheap as they used to be. I think PT has the channel seals.

 

Might just replace them.