The ones that have a numbered temp dial make it easier to get the temp right without having to keep adjusting it after waiting for the water temp to adjust and the thermometer to read it. I have one with the numbered dial which is a Theo by Hydor which I like a lot. and then I have another forget the brand that gives no indication of temp on the dial. This was a pain to find the right temp setting. You may also consider one that has an auto off if the water drops below a certain level, I believe most do this though, but it is definitely a good safety feature to look for.