I had a logitech performance mx and then bought another logitech g500 for gaming.
I have a relatively small hand so my experience might be different from many other guys.
The only problem I have hand with mx is the weight. it's wireless so needs a aa battery, which is pretty heavy. It's big, too, but that's not a big issue.
When I shifted to g500, it's good in resolution, I think it's 3000dpi or so. I don't like the grip, it's no big deal as before. The adjustable weight system is pretty good though I always keep it as light as possible.
The big problem is the left click key on g500 gives crazy sound, from the first day. It's pretty annoying high pitch sound. I guess I will request rma in a while.
On the other hand, you can adjust lots of params with the given software of g500, which is pretty cool.