We have used Mixpanel to gather and analyze analytics for a number of clients. It's easy to use and set up. Recently, I evaluated Heap Analytics for one of our clients and it blew me away.
Where Heap and Mixpanel both win
Both allow you to drill in to the sessions of individual users and see what they are doing on your site.
For Mixpanel, this is an additional product and, as such, is an addition to your bill, but it's hard to say which product or products is more expensive because they have drastically different pricing models.
They both allow you to do great segmentation reports.
They both have iOS support.
Where Heap wins
Heap automatically records everything your users do.
In most cases, this allows you to retroactively add metrics and quickly answer questions you have right now. Watch the video on their homepage to see what it looks like. You no longer need to instrument some things, deploy, wait and then get an answer to your question.
Tight feedback loops are important, and Heap lets you change your mind about the questions you want to ask and get answers immediately. Mixpanel requires foresight and patience or an "instrument everything" approach which they discourage with their per event pricing model.
Heap lets you see what users do even if you haven't added an event for it.
Because it records everything they do, you can see what they click on or which text fields they change. You can then easily turn it into an event.
Heap has a slick "Event Visualizer" that lets you add events by clicking on actual elements on your page.
Think Google Analytics' overlay view, but for defining events.
Heap is easier to use in my opinion.
I've butted up against a number of confusing and not-so-straight-forward experiences with Mixpanel. The Heap api is simpler as well, there is no
alias discrepancy, just a single
Heap doesn't penalize you for tracking lots of things.
They charge per session, so it doesn't matter how many events you have.
Heap records everything, even in iOS.
I haven't used it, so I cannot speak from experience, but I know I would rather have automatic instrumentation than needing to pepper
[mixpanel track:] in all of my handlers.
Where Mixpanel wins
Mixpanel People enables you to send smart, targeted emails.
If you've ever received an email that said something like, "Hi, we noticed you signed up and created a project but haven't added any tasks yet. Here's a quick video on how to add tasks!" then you know what I'm referring to. You can set up emails to send automatically to people X time after they do Y thing Z times, a handy thing for encouraging engagement.
Mixpanel lets you analyze retention.
These reports are helpful to see what features keep people coming back for more so you know where to invest.
Mixpanel lets you see funnel conversion plotted over time.
With Heap it's all or nothing, though both allow you to use segmentation to accomplish similar things if you're interested in the effect that a tweak had on conversion rate.
Mixpanel lets you compare multiple events.
You can select as many events as you want and plot them on the same chart.
Mixpanel lets analyze formulas like ratios between events.
You can build formulas, see them charted over time, and even compare them to others.
Mixpanel is fast.
When you want to see a report, you can, usually in less than a couple seconds. Heap seems to get slower as it gets more and more data, reports are now taking 5-15 seconds to load, though after contacting Heap support they were able to allocate more shards to my account, which sped things up considerably.
Where my money is
If I were a betting man, I would guess that older analytics companies, including Mixpanel, are going to need to take a strong look at what Heap is doing and imitate it as soon as possible or be left behind. Recording everything is just too powerful, especially for bigger apps that have a large surface area.
Unfortunately though, Heap is still missing some things that really make Mixpanel a powerful tool. If Heap were to catch up in feature set and performance it would be a force to be reckoned with. Until then, Mixpanel is probably a safer bet unless you do not need the features it provides.