What route is that? Did the OP say where he is flying from?

Since this is for biz, I would be inclined to go with UA just because of the greater liklihood of making your flts on time. That's the primary reason my wife flies CO to IAH.

OTOH, flying DL might be appealing if you don't mind connecting through ATL and can allow additional time for it. Those extra miles can help, especially if you are scrambling for status at the end of the year.

As for Plat status, it is second tier for DL but third tier for UA.