electronic tuners people use
General mountain dulcimer or music discussions
I've had a few of the easy clip-ons and find them mostly all the same. I just bought one of the new Snark SN-8 (new black) models on Ebay and only paid $8 or $9, so look around and don't pay too much. I must like it since I have two of the SN-2 (red) models. The only complaints I've heard about the Snarks is that sometimes the head snaps off the base. But that's got to be user error, so if you take care of it, it will be fine. And I don't think you can beat the price.
There really aren't a whole lot of variables. How fast and accurate is the tuner? How easy it is to read? Other than that, some have a crappy metronome built in and most allow for calibration, so perhaps you can play along with a piano or autoharp or hammered dulicimer that is tuned to itself but a tad off. I personally never use those features.