I think the collection size is a valid point. I think IM is better, but the one thing with IM is that it really only has one pace. You either own it or it owns you and it's full speed from the plunge. If that's what you're stuck with all the time you run a risk of it getting old. At least with Met you can vary the pace and take it easy every now and then if you want. I have both, love both, and sometimes I walk past IM and just say, "No, I'm not fighting you today."