I need to extend a LAN between two buildings. The distance is about 100m. I cannot mount outside, they will sit inside on the windowsill on both sides, there is line of sight between the windows. I'm a bit concerned they may be moved or bumped so I am leaning towards omnidirectional models.

I'm looking at a pair of OmniTik 5AC units to bridge between two buildings on 5 GHz dedicated channel, and then access points on each side, (like a hAP AC Lite) to serve clients in each location.

I will also need some sort of tabletop (windowsill-top...) mount for the Omnitik units.

Any comments or feedback?

Thanks!

I wouldn't use an Omnitik for that. I'd use a wAP AC at either end, and then whatever APs you want inside the buildings.
If I could mount it outside, I'd use SXTs.

Thanks, can give more details on your recommendation? I looked at both models, the OmniTIK has 7.5DBi antennas whereas the the wAP AC has 2DBi antennas, so I figured the OmniTIK looked better for dedicated point-to-point over 100 meters or so. But I've never used either models.

The new Wireless Wire would be perfect for that - even supposed to work through windows......https://download2.mikrotik.com/news/news_78.pdf

The Omnitik isn't ideal for PtP, since it's omnidirectional. I have one on the roof of a motel and SXT's pointing to it from the other buildings. I've also got an RB911 sitting on a window sill in the house next door.
SXT's are a good choice, but you'd need to find some way to mount them. The wAPs (and the new Wireless Wire, which has the same form factor) have desktop mounts. The standard wAP isn't the best choice, but given the short range, clear line of sight and the window in the way, it's a simple and easy solution.