Admittedly, a lot is about what I am used to. I've been using ForeFlight for 11 years. But, in both cases, I think the interface is clunky compared with other EFBs. With Avare, even simple IFR flight planning is a chore. You you enter and search waypoints one by one by one and there is no ability to enter airways at all. And no real rubberbanding to make up for it.. FltPlan Go does much better in this department since it's an extension of FltPlan.com, which was at the top of the flight planning field since long before EFBs existed. So it's mostly about the flow of getting around, plus I there' are still stability issues with the Android version.Just out of curiosity, what do you consider the key features missing for IFR flight?
They are both on my phone as IFR backups because, once I am in "ultimate" EFB backup mode (iPad failure, not app failure), all I really care about is a basic moving map and the ability to pull up an approach chart. These are good enough for that and I have both because of the stability issues I've seen in Go! and the limited functionality of Avare. My preference is Go! because of the ability to easily create or change a route, but I think Avare is more stable in a pinch.
Or maybe I'm just biased against free for an essential tool.