Both are score first, small guards (I've seen a lot of cope from Rockets fans that Green is somehow 6'6, as opposed to being closer to 6'4), that can't defend or playmake very well. Jalen Green is probably slightly better at playmaking, and more athletic.
Jalen Green last season: 34.2 minutes per game, 22.1 points per game, .338 from 3 on 7.3 attempts per game. .471 efg, 53.8% ts.
Defenses could focus in on him, and he was on a team with little spacing.
Cam Thomas last season: 16.6 minutes per game, 10.6 points per game, . .383 from 3 on 2.3 attempts per game. .497 efg, 56.7% ts.
Personally, I think both are bad archetypes that rarely contribute to winning. Investment cost in Jalen Green has been much higher. I would say right now, Jalen Green only has the promise of potential. Despite playing more minutes, and shooting more threes and scoring more points, he is inconsistent and does not offer much value.
This isn't to say Cam Thomas is much better, however.
Better question is Jalen Green or Jordan Poole