They are almost the same thing. The big loop on the back (for guide mode) is oriented horizontally on the Reverso and vertically on the Guide, and the little loop on the front of the Reverso is designed to take a carabiner (but only a Petzl carabiner), whereas a carabiner won't fit in the equivalent hole on the Guide. Really that hole is redundant, because you can just pull on the biner that the rope runs over to unweight, so that's not a huge advantage. The Guide is slightly stronger, but the Reverso's slightly lighter. I personally prefer the Guide because I think it works better as a normal ATC and offers more control on fast rappels, but I've used it more. It's your call.To add/amend Angus' comments...don't just pull on the biner to unweight the system, it is setting yourself up for disaster. One really needs to learn how to use these devices properly (instructions and experienced users) because there are reasons why these holes exist and why they are certain sizes. I have come across too many users applying these devices wrong over the years, so be careful and get trained. That being said they are extremely useful!!!to add to the previous two comments- i've been using the reverso3 at the gym and it feeds a lot better than the atc guide- particularly w/ thicker ropes-

If you haven't already bought a medium, do it. I'm 32 waist and a med fits just right over a pair of shorts.for others considering this question i wear 28/30 and the small fits just right over jeans-

i have a 30" waist as well but the small fits perfectly for me- i can't adjust the waist any tighter- if i were to go w/ a medium i feel as though it would be loose even w/ the waist strap pulled completely through-it seems as though it might depend on your body type along w/ waist size- i have a lean body type- that may help w/ a decision-