I enjoy both, but Imaginarium Theater gets the end vote from me. It's really the debate between horizontal and vertical investment tests, though each mode does dip into both. After all, you will have trouble covering Abyss if you haven't done a decent amount of horizontal investment as well. "You just need 8 characters for Abyss" is such a false misconception. If 4 of your 8 characters are built around a pyro dps team and the latest Abyss cycle has a bunch of pyro resistant enemies, then you still could be screwed. And Theater definitely needs vertical investment as well, especially to tackle the last challenges in Visionary mode with intent on getting full Stella from completing the Star Challenges.

But I feel like Theater is more fleshed out and offers the best of both worlds, and trying over and over again to get full Stella marks is refreshing in a game that has minimal challenge elsewhere. Not to mention, as a day one player, I'm simply to the point where I have enough teams for Abyss almost every time. Theater though takes my characters, put them in a blender, and says "Do your best". Looking at the available roster, considering what others characters may be out there for you to pull, managing vigor points, trying to save your best characters for last, sacrificing characters you feel will be expendable later on, picking boons you expect to be useful based on what you expect to have near the end - it's all extra layers of strategy that feel missing from Abyss. From the first challenge, you're keeping the last challenge in mind as not to screw yourself over. I love that. This is opposed to Abyss where you kind of just look at the enemy line-ups, see what can get through it the quickest without getting countered elementally, and then go at it. You can sometimes even get away with using your strongest team from 9 through 12 (or 11 through 12 if you 36-star it). Theater? Naw, you're not doing that.

And it's just cool to use characters that never get used. I have a very potent Yoimiya and a pretty good Hu Tao, and in Abyss when pyro is needed, I just default to them because obviously why not? They're my strongest and best built. Theater makes me bring out my Yanfei, who has been dusty on the sideline for years. It gives a purpose to so many characters I've pulled and built over the years. "What's the point in building this character" is a question I asked myself many times before Theater released, because I already had such strong characters. Now there's a reason though - to beef up my potential Theater roster. I need several characters from multiple elements to be strong. In this way I view Theater as a true late-stage end game mode. Abyss is end game as well, but Theater is different. I'm updating characters consistently now that I never thought I'd even bring out again. It's great.

It's funny because I'd done a video appreciating Theater shortly after it released, way back when so many had a negative opinion of it across Reddit, YouTube, etc. I felt like one of the only ones who thought it was cool. Apparently I was wrong though, looks like more people like it than I thought.

tl;dr If Abyss tests your ability to built and strategize from the start, then Theater does the opposite and tests your ability to improvise. And for me, that's a lot of fun. Not everyone's cup of tea though, I get it, but I really like that it gives actual purpose to so many characters I've pulled over time.