If you have a MSR Simmerlite, I would probably just stick with that. But that is probably because I am cheap and wouldn't want to buy another one. With that being said, I really like the Jetboil Zip because it only weighs 12 ounces and is really compact which makes it nice to take backpacking. The simmerlite is really lite too though. I haven't had a Simmerlite so I can"t tell you which one is better but I can tell you that I have had a really good experience with my Jetboil Zip
. It is more fuel efficient and can boil half a liter of water in just over 2 minutes. The quicker you can boil water, the quicker you don't have to have a stove running in your tent. I would avoid cooking stuff in your tent if at all possible but I would think the Jetboil Zip is better for that.