Tough decision. I don't think it depends so much on how she compares to her current teammates. I think the more important consideration is what type of 14u teams you plan to compete against next year. If you're asking a typical 6th grader to play against a group of VERY GOOD freshmen-to-be kids, I'd say no way. Notice I said "typical". I never would have let my DD as an 11 yr. old play up against a team with equivalent talent to her 14u team. She could have held her own against an average 14u team, but they were NOT playing average teams.
Admittedly, the Slammers are a very good team - for their age group - probably currently the best in Ohio. BUT, when playing up a couple of age levels (as they typically do), it is NOT against top level teams in the state - and Bill knows that. That's no slam (pun not intended) against them, because what they have accomplished to date is rarely attempted by anyone else.
If your DD pitches, be aware that a GOOD 14u can hit the ball MUCH harder than an 11 or 12 yr. old. That's about the only real danger. All things equal, those two years from 10 to 12 make a big difference. At 14, girls are starting to develop muscle mass and quickness that is not there at 12. Now, if she's playing in a lower level tournament against teams similar to a freshman high school team, there's much less to worry about. Only you know your daughter's skills, and it's one of those tough parental decisions.