I've used both. Mixpanel is much, much easier to setup and use. Plus it's more helpful in understanding where your bottlenecks are.

Ok let me explain myself. MP is very easy and reliable to use, but the problem is when you get into their dashboard. It's hard to analyze your data since there are no reporting tools. In Analytics you can create really detailed reports to segment your data and measure against certain metrics, in MP you just get basically a click logger. Also my gripe with the funnel setup is that you can create an unlimited amount of funnels, but you cannot delete them. You can only view your funnel by week-duration and no other time frame. There are also no built-in baseline metrics. If you want to track user's browser, OS, or whatever, you have to log those yourself. Some might consider that a feature, I didn't enjoy it.

Sounds like you are a better fit for Google Analytics and Optimizer.