TromboneChat

Looking for a good tenor/alto double case, between the Marcus Bonna double, Bags of Spain double, and K-SES double right now. Anyone have experience with any of these or know which would be the best option?

I know lots of good players with the Bonna.

Used to have the Bona, sold it about ten years back. Have the ols version of the Bags of Spain currently.
The Bona is a very good case, though I don't love that the alto is under the tenor since that's the one that always comes out. But extremely well built and the horns feel very safe. Not very big for a double.

The Bags is smaller, which was the selling point for me as I travel a lot. Like somehow the diameter is barely bigger than the tenor bell and it's barely longer than the tenor slide. It's also much lighter than the Bona as well. The tenor and alto are on either side, so you can get one out without taking out the other. I used to have a Lidl rotary bass trumpet and that fit on the alto side, so it provided some flexibility. If you're traveling without an alto, the alto side can double as your, very small, suitcase. However it's a bit sketchy if your alto has a valve- I've got one with a trill valve. you've got to be careful when putting it in so you don't mess up the tenor, which isn't a worry with the Bona. It's just very compact so you have to be conscious when putting stuff in there. I believe the newer ones might be slightly bigger, but I'm not totally sure.
It zips together with a heavy duty zipper, which is nice quality, but the two halves have some flex because of it which worries me slightly. I'd be a bit nervous if I had to gate check it, but it's also quite small so it wasn't ever a problem. I'd say it's definitely got more flaws than the Bona and is a less durable case, but I prefer it due to its size and separated horn access.

Would both of those be fine fitting in a standard overhead bin on a plane, or just the Bags one?

I've gotten them both in overheads on larger planes (us to EU or larger cross us flights) no problem. I've only taken the Bags case on a very small plane. It fit on that, but was a squeeze.