It's 6U... which is not to downplay it at all... but the considerations are a bit different than if she's 13. Does she have more friends that play softball or baseball? Is is a co-ed req league, or will she be playing with all boys? Is it more convenient for the family to be involved right now with this league?
Main point is to have fun and, if possible, learn a little bit about throwing, fielding and catching. Either version can be very competitive, all things considered, or just a fun afternoon out in the sun. Just depends what is the best fit for your kid and family.
I'd definitely say that if your daughter wants to play competitive softball as a teenager and possibly in college, she needs to be playing that sport by 12 at the latest or she'll have a tough transition. That's just my experience, but I'm sure there are plenty of exceptions.
Most of all- ENJOY THE TIME!! They grow up quick.