Funnel analysis is Mixpanel's bread and butter: we offer instant, retroactive funnels. You can create funnels on the fly, control the time window to completion (1 session v 1 week v 1 month), and understand how your conversion rates are changing over time.
You can also cohortize your funnels to ask questions like, "Did users from campaign A convert better than campaign B?"
Finally, you can create "OR" funnels in case you have, say, two ways to browse items, or two ways to move from viewing items to adding to cart.
KISSmetrics restricts your ability to dig deeply into user cohorts, has no engagement platform to act on your insights (we offer email, push, mobile in-app, web in-app, and surveys), and is web only.
Ever planning on launching a mobile app? You'll probably want to understand your users' cross-platform experience :)