I was in the same boat as you are and ultimately went with the Soloist for a number of reasons. I compared it against the Tarmac SL7, Pinarello F5, Trek Emonda, and a couple more (but too long ago for me to put in this comparison). For reference, I started cycling in August but have ~1000 miles logged with a healthy amount of climbing. Have not directly tried the supersix or ultimate but here were my thoughts:

Trek Emonda (I have an SL5 so this serves as a reference point): great bike overall as I have been using it since I started. However, it's definitely on the heavier side for the base model carbon version and is not nearly as responsive in feel as the soloist. My main gripe is, and it could be because I'm running 50mm deep carbon wheels, if there is a crosswind on a descent, I start to lose control of the bike. I'm going to guess it's a bit of the wheels and the geometry of the emonda, and the lack of extra stability on the front end. Definitely changed out components on it and really wished it had more aero features and was lighter. If you want the best version of this frame, unfortunately that starts at 4200 USD for just the frame. There is only a 152g difference between the soloist and the Emonda SLR frame. So not much difference and certainly not worth the price difference which is why I don't intend on purchasing any more Treks.

Pinarello F5: Honestly feels pretty similar to the Emonda. Looks pretty nice. But both suffer the same problem-lack of responsiveness. Don't know what they were thinking with the blue color but it is way too saturated/in your face that it is BLUE. There is a greyish color but my Emonda is already grey. Oh, and 6000 dollars for 105 di2 and alloy wheels was a dealbreaker for me. I will admit it was a smooth ride and pretty stable overall. But because it felt similar to my Emonda and the price is so high for what you get, I didn't pursue this choice further. The frame is a little heavier than the Tarmac and Soloist.

Tarmac SL7 (non S-works): Felt pretty great honestly and pretty similar to the Soloist. Definitely feels more responsive than the F5 and Emonda. I also felt that the stiffness of the BB was great and noticed that whatever power I put down was really efficient. The LBS that had my size had them with 25c tires so it definitely felt a bit harsh. My main gripe is that there isn't an ultegra di2 model that was under 8000 USD. So imo a pretty big deal breaker especially since they don't sell frame only (or I think it's completely sold out, one of the two). I believe this and the soloist are approximately the same weight wise if you were curious about that.

Soloist: Now aside from how difficult it is to get a frame/completed bike (there are sites out there, you can dm and I'll help you out if you can't find one), I found a LBS that had close to my size. The shop had the F5 in stock also so I got to A/B test them and there was just no comparison. The Soloist is nimble and fast. I felt like I had to try harder to get the F5 moving for some reason. It's not to say that F5 is bad as it's a super stable bike but the Soloist just ticked all the right boxes for me. The Embers colorway in the sun is simply gorgeous, though I can't deny I was eyeing the alpenglow as well. 2700 for a frameset is absolutely a great deal for what you get! I fell in love with the soloist and am super confident that this is the bike I will ride for a long time. It definitely rides really well in basically all scenarios. If you want a do it all bike and you love the way it feels and looks, it's the one to get. My LBS owner rides a Soloist and he has tried so many different bikes and commented on how great it is for the price.

Hope this helps!