Most Helpful Guy
At the end of the day, find someone who has a mixture of both. If you want to try things out, i will say go out and observe guy A for longer time. Looks like he knows what he's doing that's why he has had many open relationships. We can't 100% sure he will sleep around when he's being exclusive with you.
Most Helpful Girl
A guy's amount of relationship experience wouldn't even be a factor in my decision. I would choose the guy that I was most attracted to, who I have more in common with, shares my values/ideals, and who I feel more of a connection with.
If you like the second guy more then I think there's nothing wrong with you being proactive and taking the initiative to ask him out. I wouldn't just accept the first guy's offer if you don't feel as much of a connection with him.