I was going to suggest the SPG (Starwood) Amex as the best choice, as the airline transfer rates are much more favorable than from HHonors, but then I noticed that United is one of the few 2:1 transfers.

Still, that might be a good route for the OP to reap a choice of hotel and/or airline rewards since SPG does redeem at 1:1 with USAir as well as 1:1 or otherwise favorable rates with other Star Alliance partners that could be used for United award tickets. (And that's really 1:1.25 with the 25% bonus on transferring 20k points.) $30/yr for the SPG card, and you get SPG Preferred Plus status (Gold-level upgrades, 4pm late checkout) as well as a yearly SPG50 50% off cert. Definitely better than the Hilton Amex as far as rewards go if you're not certain you'll be using them for hotel stays or for airline awards. (And SPG does cash+points awards which is rather nice. Of course Hilton has a lot more properties, and more value-oriented brands, so that might need to be taken into account; but the airline transfers are much less favorable.)

If United miles are really preferred, though, then a United affinity card would be the best route. There don't seem to be any programs that transfer well to United; Amex MR, Diner's Club, and HHonors all don't support United, and as noted SPG is at 50% point value.