If I were to have only one, it'd be the parking sensors. As long as they're placed properly on your bumper (i.e. not all in the middle like in other people's cars), they'd do a lot in assisting you as you park. Always keep in mind that these sensors are supplementary only, you should be able to park even without them. It shouldn't be an excuse to be lazy.

Anyway, I prefer the parking sensors over the camera because it's more intuitive to use. Before the days of rear view cameras, everyone had to glance through his rear windscreen to see if his parking is good enough. Pretty hard to do that and look at the HU at the same time right?

Of course, if you can afford both, get both, though I wouldn't suggest getting a camera without the backup sensors.