I'm also coming from the 4 XL and have had my 6 Pro one week today. To me, it's a worthy upgrade. I've never used the regular 6 so I can't speak to that. I was never a big fan of the face unlock. It was pretty quick and mostly reliable, but I'd say I have about the same success rate with the 6P fingerprint scanner, around 90-95 percent. With the 4 XL, I was often having to reposition the phone directly in front of me, or move it further away from my face, etc. So whether I'm having to reposition the phone in front of my face or reposition my thumb on the screen, it's all the same to me. And with the fingerprint, I have the ability to unlock the phone if I'm holding it at an odd angle or if it's sitting flat on a desk. Overall, I greatly prefer having fingerprint available. As for Soli, the only thing I really used it for was snoozing alarms, so I don't miss it. Same for the squeezy sides, never used it.
I feel like the 6P speakers don't sound as "full" as the 4 XL, but they're perfectly fine for the occasional YouTube video, etc. Much better than the abomination that were the Pixel 5 speakers. I sent that phone back after a week and went back to my 4 XL due to the atrocious sound, especially phone calls. Calls sound great on the 6P, just as they did on the 4 XL. To me the beautiful screen (my 4 XL always had a slight green tint...no green tint here), 120 Hz, faster storage, and more RAM more than make up for a slight perceived downgrade in sound from the loudspeakers. Even though the dimensions are similar, the 6P does feel larger and heavier than the 4 XL, but I've already gotten used to it in just a week. If you're coming from a 4 XL and considering the 6P, I say go for it. The regular 6 might be a different story, though.