I personally have the 15 key mk.2 (regular) and it's great. I had the 6 key (mini) which was fine but the real estate on the regular is much nicer. I haven't played with the XL or the newer stream deck+, but the + might be a nice compromise between the mini and regular along with adjustment knobs.
The XL would certainly come in handy if you didn't want to pack functions into folders as much, but I find that the folder functionality helps lock down similar workflows. For example, I have a work folder that has shortcuts, hotkeys, and general quick access items just for work, all within one page. I also have a lights folder that contains a bunch of different smart bulb configurations for different vibes.
For me, the mini is too small, the XL is too big, but the regular is just right. I like the idea of the + but it's likely more useful if you find yourself regularly adjusting audio levels in a stream, discord chat or just for your own preference. The + knobs can do more than just adjust audio however, and if you find yourself with use cases when you're fine adjusting settings on the fly, the + might be for you.