I like a digital for doing a quick check of the temp when I do a water change. It's usually in there for only 15 minutes (all Ralph needs is 30 seconds to do the damage

).
I've got a sticky one on the outside for quick ref on the tank, but it takes too long to re-adjust and isn't entirely accurate.
Emailed Zoo-med last night, and already got a response!
Thank you for contacting us. The Aquatic Turtle Thermometer should be stuck to the side of your aquarium using the provided suction cup. Should it some lose it should float. Please let me know if you have any further questions.
So if I can find one of those "aquatic turtle" ones for a decent price (lets not forget I'm up here in the Frozen North, and most online US companies charge $20+ for shipping!) I might go for that one. If not, I'll look into the ESU one, Jozzep and someone else mentioned they use that one, so it might be pretty good!