I literally spent the entire weekend debating the two options as well. I can tell you what my research turned up.

For the Orbi, my biggest concern was the ping because I am a gamer and that matters to me. I found mixed reviews online so it wasn't abundantly clear what my best option was. There's also the fact that the Orbi isn't a "true" mesh system, it's a "hub and spoke" system.

The issues with AiMesh seem to be that it's not totally user-friendly. It's not a fully-fleshed out technology yet, and while it has the potential to eventually be better than traditional mesh systems, it's not there yet, and apparently there are issues with the firmware and the software. There's also the fact that while AiMesh is a FEATURE of the ASUS router line, it's not the primary function, while the Orbi was designed specifically to be a "mesh" system.

I literally spent all of yesterday reading reviews and comparisons and chatting in forums, and I ended up picking up the Orbi last night. So far so good. I live in a 3-storey home plus basement and so far all of the dead zones that I had in my home with my old setup (Linksys AC1900 in main floor living room plus Linksys Range Extender in my 2nd floor office). I put the Orbi router and satellite in the same locations. I have a Chromecast in my 2nd floor bedroom which would always lag and now it runs flawlessly. My 3rd floor is family room with my entertainment centre, and my Xbox would always disconnect from the Internet, but no issues so far (again, it hasn't been 24 hours yet, so still testing). I haven't tested my shed (maybe 40 feet from my house), my garage or driveway yet. If I begin to experience issues I may return it, but for now it's working great and it really is noticeably faster in some areas.

I will say, if you go the AiMesh route, don't splurge on the AC5300 Rapture. Benchmarks showed that the cheaper AC86U performed better, although not as many advanced features, so I'd pick up a pair of those. I'm in Canada so if you are in the US I don't know if this will apply, but there are BF deals here for both the Orbi and the AC86U's this week.