Hotel cards are generally not valuable for spending on the cards, except for staying at that hotel. Their value comes from signup bonuses, renewal bonuses, and status. (See Hyatt, IHG, and Club Carlson.)

As Milesgator mentioned, the Marriott card gives a nice number of points to start, the Category 1-5 certificate on renewal, and enough nights to get Silver status. Silver's not worth much, though. Most importantly, Silver members do not get free breakfast or Internet. Still, the free night certificate is generally worth the renewal fee.

Hilton has two cards that award Gold status, which includes Internet and continental breakfast. Unfortunately, neither card seems to offer yearly renewal bonuses. The Reserve card gives two free weekend nights for signup at any property, which is a great signup bonus. Nevertheless, unless you're going to use Gold status benefits, I don't see a reason to renew.

If I had to choose one ... hmm ... if I had plans to use the Hilton certificates in a fabulous location, I'd get the Hilton Reserve. Otherwise, I'd get and keep the Marriott card, using it for Marriott stays. Hilton hotel nights tend to require more points than Marriott, so if I was looking to accrue in one place, it would be Marriott.