Just curious, but what makes real time so important to you? What do you do with your data instantly that wouldn't be better served rolling up over a longer period of time to remove any noise-iness and natural variability in your data? I'm genuinely interested in your use case.
I've also heard that KISSMetrics is more about identifying people rather than data points, so it makes retention and user-based events easier, but it seems mixpanel does that too, so I'm not sure.
KISSMetrics, visually, kicks ass. What sucks about it? It's not real-time. No simple real-time data export/querying API.
The latter is nearly a show stopper. I mean, really? No export/querying API?