I like the Snark too, but it's the only one I've had extensive experience with. I helped my old boss at his store a little last year, so I got to play around a bit with several. I bought a Snark because it worked well and was inexpensive. Reasonably rugged if you take normal care of it.
As for using open strings versus harmonics (and this applies to any tuner)- The octave harmonics (5th and 12th fret) ARE exact octaves if your strings are in good shape, and if your strings aren't in good shape then intonation is out no matter what you do. So, using harmonics is NOT in any way a problem- if the harmonic is in tune and your strings are in good shape, the open string is in tune exactly the same.
And, I'd say that the harmonic, because it's a higher frequency, gives the tuner a better sampling rate to get an accurate count of the vibrations- so in fact the harmonic probably works better for any tuner that's counting frequencies.
John