Apologies for going a little OT, but maybe this will help pinniped.

We stayed at the Petra Marriott for 2 nights in May 2011. The Marriott is well away from town, and it is far enough away (and uphill) that you cannot walk to the entrance to Petra. A few comments about pros and cons of the Marriott Petra:

1. Whatever the category listing is on the Marriott website, subtract at least one level. For a Marriott, we were disappointed with our room. A Fairfield in the USA would have been nicer.
2. We did enjoy the sunset over the Petra mountains. The Marriott is ideally located for this. Very pretty in the evening as the sun went down.
3. The hotel has a nice dinner buffet, and the price was reasonable.
4. If you have not been to Petra before, understand that it is a FULL day. Petra is fabulous, and you will probably want to see every bit of it. My wife was positively exhausted when we left. A hotel near the site entrance will permit earlier entry and allow you to collapse in your room at the end of a long but wonderful day of sightseeing.

We were on a private tour in Jordan, so our driver dropped us off in the AM and picked us up when we left the site. If I were to visit again, and did not have a driver, I'd probably pay the money and opt for the Movenpick. I know this is FlyerTalk heresy, because I don't have any "Movenpick points," but the Movenpick is very near the site. (Be careful - if I remember, there may be 2 Movenpick Petra locations? The one I'd want is the one very near the site entrance.) You said that you're going to have a rental car, and that would make it possible to stay at the Marriott and drive back and forth. I can't tell you anything about parking at Petra, however, because we didn't have a rental car.