I purchased both BG3 and Solasta the instant they were available on early access. They both are isometric turn based games utilizing a (mostly) 5E D&D ruleset. That's... where the comparison ends, in my opinion.
Both games are extremely enjoyable. I finished BG3 once already and am waiting for a few more patches/updates before I jump into it again. Character generation is enjoyable and varied and the writing/story is, in my opinion, excellent so far.
Solasta is something special to me, though. I haven't played a game that made me feel anything like this since Pathfinder: Kingmaker, just in sheer "zany high fantasy adventure" without too many excursions from the tropes that made my early days of D&D so fun - you can pick any personality or alignment and your characters will act out those personalities in conversations and banter; it's been fun so far. It's very, very trope-y - you literally start as an adventuring band in a tavern and then are sent out to explore some ancient ruins. It doesn't get any more basic D&D plotline than that, but it's done very well. (A good steak is pretty basic, but it can be cooked decently or destroyed by a bad cook; this game has what I'd call "good cooks" ... - they take basic elements and make them very enjoyable.)
Right now I have more hours put into BG3 mainly because it came out first. I honestly don't know which game I'm finding more enjoyable, but I *will* say that the raw combat and implementation of D&D rules is much more well done, to my tastes, in Solasta. BG3 probably has better writers and a lot more content in the game thus far. But man... 12.8 hours of playtime in Solasta, 65 in BG3 according to Steam; and I picked up Solasta this morning so you can guess what I've been doing all day. I put it away for the night and I can't wait to get back to it tomorrow.
Since really meaty in-depth turn based combat is my jam, I am liking Solasta a little better, maybe. They're both excellent EA games that have provided many times over their cost in terms of entertainment value. I can see myself spending more time in Solasta, though, especially when they come out with more content/campaigns. It just scratches a D&D itch that Larian's "Every tactical combat field must be filled with exploding elements, pools of ice, fire, acid, or otherwise environmental hazard..." - that just reminds me of Original Sin 1 and 2, and I wasn't terribly fond of that mechanic in those games, either. We get it, Larian. You like environmental effects.
I like pizza, but not for every meal.
But that's kinda a nitpick. Both games are great. Both are very different ways of doing a turn based 5E game. One has a huge studio behind it, one - not so much.