So - I have thoughts.
Chelsea’s academy has been quite underwhelming recently but one thing they have gotten down to a fine art is having a promising young defender breaking through every couple of season. Fikayo Tomori, Trevoh Chalobah, Marc Guehi, Levi Colwill and the latest is Bashir Humphreys (Brodi Hughes or Travis Akomeah will be next).
Anyway, I agree with you that Levi Colwill is the most talented of that group. I’m actually surprised that Chalobah is the only one there this season, he’s done pretty well but he’s probably the least talented football on that list.
However, Colwill hasn’t been consistently good at the Premier League level yet, or at least he hasn’t been consistently involved. Only played twenty-one times for Brighton, sixteen in the league, this season despite playing well in those games.
Marc Guehi is about three years older but has been a first team regular since he arrived at Crystal Palace - he’s played over eighty times in two years there. He’s also the vice captain (I believe he’s now the de-facto captain actually) and - although it’s hard to judge CBs statistically - shows up nicely in possession
He’s more of a sweeper style defender, plenty of pace, modern type defender.
However as I said above I think Colwill is more talented. He’s another step up on the ball and better aerially while having many of the same positive traits as Guehi. However you have to account for the fact that Brighton have often this season felt like he was better in a backup role than as a starter - which I think is just an experience and maturity thing. He’s clearly worked his way into the rotation more regularly recently. Colwill’s FBRef page is ridiculous (small sample size warning).
I’d say that if we wanted a full time starter right now, then Guehi is the better option. If we want someone to come in as a backup and grow into playing time then Colwill will become the better player over the next few seasons. Both are left footed and I think both could work here but Colwill is just a slightly higher level prospect despite being a little further behind in his development.
Hard to fathom what Chelsea are doing though. Could have spent no money and had Tomori and Guehi starting tonight. Instead they have Fofana and Chalobah. As seems to always be the case with Chelsea recently , they spent a tonne of money to get worse.
Having said that I’d surprised if Chelsea let Colwill go to us. I said in May of last year that most clubs would integrate him to the first team and Chelsea are probably in a position to do that next season. Although they do make dumb decisions with their youth prospects so maybe.