I've heard that many people prefer the MX2. (I may be wrong but I think either @jj1000 or @AJK said they prefer MX2?)
I've never used the MX2 but I think the back/forward buttons are a major advantage on the MX3.
The gesture button on the MX3 is a bit akward to use but I believe a lot of people had issues with the easy to use (entire thumb rest) button on the older versions not working properly.
Accurate.
What do you prefer about the MX2?