Well, I dont really need neither of them, but... from my perspective, they are doing the same thing.
You are creating a group of websites in both. Tab stacking is faster, cause you can just drag and drop and create a group of tabs, and with workspaces you have to right click on tabs and sent it to new workspace... again, to create a group of tabs. So, whats the difference here? They are the same to me.
Anyway, my point is, that tab stacking is already implemented in chromium, and the fact that it didnt show up in Opera, and instead we got workspaces, could be related. So, maybe Im wrong but I think they just reworked this feature.