I don't have my Teratorn yet so I cannot give you my first-hand opinions, but maybe a few points that I picked up over the past year that could resonate with you either way.
I initially went with the 3600 + R3 bags for last season, then traded up for a 5000 because the 3600 was just a hair small for all my winter gear. If you're never packing everything in, this could be irrelevant. If you are packing in your entire base camp to then unload and only run the bag compressed, then you still need the full volume.
I don't have different fine-tuned items for different seasons, nor do I have the budget to chase best-of-the-best, hunt-specific bags and sleep systems, and weight. For me, fall gear will be largely the same as winter and spring. I have some dietary restrictions that make a lot of backpacking food a non-starter (plus I'd rather just eat what I do normally), so almost everything I packed was real food and that added bulk.
The difference in compression between the 3600 and 5000 was basically indistinguishable, whereas the 7200 did look to be a hair much for me in the main bag compartment. The K3 6400 to me seemed about perfect in this regard, probably similar in practice to your 5900, which is basically the same size as the Teratorn. I have no experience with this, just my subjective opinion.
The horseshoe pocket was much easier to access the bag from either side on the 5000, so I'm glad the Teratorn retained that feature.
The only thing that really annoyed me about my 5000 is that Exo only puts single zippers on everything but the main bag access. I understand the mindset but if you have the bag compressed and want to use the side zips or front stretch pocket, you have to dig up under the lid to open them, or pop the lid buckles.
Little things like that seemed insignificant, but after doing that over again while not finding animals for days, lack of sleep, and calorie deficit, they really start to get annoying...at least to me
I'm glad the Teratorn forewent the side pockets, as I could not fill them without poaching volume from the bag interior, or making them puff out and force the R3 bags out pretty far unless really cinched down.
The biggest advantage of the 3600 + side bag route is they are very easy to completely remove if you just want a small, slick setup, and the 3600 is that. Folding the Teratorn wings behind the frame would accomplish the same, but I get the idea of just removing when they'd never be used.
I also get that the 3600 is just a good looking bag. It looks freaking cool and size-efficient, whereas the Teratorn looks a bit more...utilitarian, but definitely more usefully-efficient.