Depends on what one values. I tried TST and Sidebery among other extensions for Firefox, but their "bolted on" feeling always bothered me. That can be somewhat fixed with userChrome mods, but those tend to be fragile and it's not unusual for them break with updates, and even with the mods they're not as integrated as native vertical tabs.

For my personal usage I find the deep nesting of TST to get in the way more than it helps and so the simpler model of Arc/Zen where nesting only happens intentionally is nice.