I have been using the DSLR / GPS combination for several years as a way of mapping rare plants. By having my full track I can buffer it to make a survey polygon and then I buffer the appropriate photo locations to make an “occupied” polygon. This allows me to map multiple species in a single survey even when they partially overlap.
I have several small gps loggers but my favorite is the Holux M-241. It has a display that shows the time to seconds so I can sync my photo times very accurately by taking a photo of the clock. I’ve been keeping track and my Pentax K-x clock drifts about 30 seconds per month while the K-70 clock drifts by about 10 seconds per month. The Holux will also run for about 36 hours on a single rechargeable AA battery, my phone can’t even come close in remote areas where it’s struggling to find a signal.
I use Picmeta Photo Tracker to geotag after I’ve adjusted the photo times in Picasa. Picasa has a nice feature that lets you adjust the time of a group of photos by setting the actual time of the first photo and it calculates the difference and applies it to the rest. If the first photo is the picture of the clock on my gps it’s very easy to get it right.