Personal anecdote: before I ever played competitive in a league and was just learning comp through lobbies (lol) someone tried to pick me up as medic for their open team. At the time, I was playing lobbies with the desire to find a UGC HL team the following season, since I had read that HL was easier than 6s for pub players like myself.
I was thrown into a 5 night a week scrim schedule and team fights without realizing it was going to be for a paid league, with zero clue how to play 6s. I backed out pretty quickly, but tbh if I would have accepted that spot, I would have learned A TON. Instead, I formed a HL team and played HL for 4 seasons before switching to ESEA.
In the end, what really mattered was how much time I was willing to commit and if I found people I wanted to play with. That's really what it comes down to - you want to improve, do you want to play 2 nights a week? 7 nights a week? Do you know anyone who plays comp already that you might be able to join a team with or learn from?
UGC in the lower level divisions (iron->silver) still take playing 6s pretty seriously. They scrim, albeit significantly less than ESEA teams usually, and there's one match a week. ESEA low open is at the same level, but the teams mostly scrim 5 nights a week with two matches a week, a significantly larger commitment.
If you want to learn fast, develop your skills fast, and start your ESEA career, then join an open team. If you'd rather dip your toes first, try UGC.