I had my daughter swing the 31/21 and 31/20 CF8 tonight. The place I went did not have the Makos to try. I thought the 31/21 was hard for her to get around. Of course, the fast pitch was set to 55 and we didn't realize it. After the high school player was done they lowered it to 45 and she did better (they had 4 cages, only 1 softball). We then switched to the 31/20 and she had more success. I had her hit with her 30/18 as well which she hit the best with. It seemed with the 31/21 the bat dropped a little swinging in a loop instead of straight to the ball.
Maybe she's not ready for a bigger stick. I'm not sure. I have some swings videoed with her using the 31/20. Not her best but it was to help me judge if the bat was too heavy.
[video]http://www.wissearch.net/videos/img_2535.mov[/video]
Which one did she like the feel of best between the two CF8s? You really can't go wrong with either, IMO.