eTrex Vista C
In Grand Canyon National Park, with its well-developed trail system and more than adequate signage, you should be fine with a good map and compass if you're staying in the front country. Of course, the backcountry poses more serious challenges (to be sure, the Big Ditch is a challenging environment wherever you wander), so you should always have the tools to find your way home. Whether that means the old-fashioned map-compass combo or GPS navigational tools, I'll leave that to you.
Beyond that, GPS units are great fun and certainly have their uses. If you're hiking off-trail, for instance, they're handy for marking the location of your tent (or car), just on the off chance the weather closes in or it gets dark. They're great for marking favorite fishing spots, at streamside or on open water, and for measuring distances traversed if that's of interest.
One very good GPS unit that's pretty high-end is the Garmin eTrex Vista C ($321; www.garmin.com). It comes pre-loaded with a U.S. basemap (proffering basic street, town, and landmark information) and can be loaded with a range of MapSource products via a USB cable. It has an altimeter and electronic compass, and can give turn-by-turn directions when driving around a city. And, it has a bright color display. All in all, this is a great little unit.
Magellan's eXplorist 210 comes in at a friendlier price ($180; www.magellangps.com), in part because of its grayscale display (it also lacks the altimeter and compass features). But it too has a basemap, can be loaded with a wide range of maps, and is PC-connectable. Consider it another excellent, all-around unit for your pack.
Either of these units will keep you posted on your location, in the woods or in the city. It comes down to what price you want to pay and what feature set seems most appealing.
For more expert reviews of GPS devices, check out Outside Online's GPS Buying Guide.