I used to carry a dedicated GPS track logger, always clipped to my camera strap, but now I just use an inexpensive app (GeoTagr) on my iPhone. Here's why:
1. I always have my phone with me.
2. Many phones, including all the iPhone models, augment their GPS location with position triangulation from cell towers when GPS signals are weak. Very helpful in urban areas.
3. The GeoTagr tracks can be read directly by Lightroom. No need for a funky intermediate file transformation, which was required with my old tagger. As long as you can get the track file to Lightroom (I transfer it via Dropbox), O/S does not matter. I use both Macs and PCs.
4. You can do other things with GPS tracks besides just tagging photos (keeping track of exercise miles for example), so something that only works with a camera is not as flexible.
The phone battery probably takes a small hit while recording, so keep that in mind. If you're way out in the boonies, where cell phones won't work, you might want a dedicated tracker device that takes ordinary batteries.