Either one is an excellent investment. I have a Garmin GPS III Pilot that I use as a compliment to everything else on the panel. I would like to get an aviation-band transciever but since I have a handheld amateur radio, I haven't really saved up for it. The one good thing about the transciever is that you can listen to air traffic control when you're driving or at home. It's good for "radio training." Don't talk on the aviation-band frequencies unless you're actually in a plane or at the airport getting a clearance. You don't want the FAA to tell the FCC to come find you.
Keep in mind that, as of this day, no handheld GPS is certified for IFR use, only VFR. I guess it would be okay to use it if everything else went down when IFR but the FARs say "no" as far as a primary instrument.
You could get a transciever for Christmas and then spring for a low-end GPS yourself. The Garmin eTrex Legend is nice but doesn't have all the aviation data capabilites. It does have a moving map that you could enter airport and navigation waypoints into though. I did that with my old Garmin eMap until I got the GPS III Pilot.