The ancestry information in Tree Style Tabs (and also with Orion's built-in vertical tabs) is an undersold feature, I think. Edge has vertical tabs and they're not terribly useful. You get a constant-sized click target, which is a huge UX bump over Chrome's shrinking targets, but having trees of tabs is amazingly useful for organization.

I hadn't really thought about the side-by-side window thing, though, so I'll keep that in mind when debating vertical tabs. I usually run with a multi-monitor and while I do side-by-side with i3, that's on a large monitor so screen real estate isn't a problem.