The 10 and the 610 seem a very odd pair to be choosing between! One is dead basic, and the other is top of the range.
TBH I'd pretty much ignore the calories burned - it won't vary very much by pace. If you are reasonably average weight then just assume 100 Kcals per mile and you won't be that far out!
If you really are only going to want to know how far and fast you have run after the event, then mapping software is free and does the job well.
If you want to know on the hoof then gps is probably the route to go down.
The question really is what else do you want & what might you want to do, but haven't yet thought of? When choosing for myself (on a limited budget), I knew that though my most usual use would be pretty standard, I would want the opportunity of setting up my own interval sessions, and also multisport use. I chose to go for an old 2nd hand 305 with the features I wanted rather than a newer, sleeker, simpler model.
As Daeve says, check out DCrainmaker's site, if you haven't already done so,as he really puts his gadgets through their paces, and thinks of doing all sorts of things with them which you might not have thought of, but might want to do once you know about them. Be prepared to take some time - he is very detailed, I had to read stuff several times in order to really take it all in. Because he has used so many gadgets he can really get into the minutiae of that particular minor feature which actually is really great, or a real annoyance. Remember though that he is a triathlete so some of the things which are important to him may not be to you.
