My daughter whos 13 has just started on a Bw Beta rev 80. She gets on ok with it but it can be a bitch for her to kick sometimes.
I have just bought myself a Gas Gas 125 txt pro to start on and she had a go on it but said she still prefered her "Bertie!!"
However the gasser is much easier to start and is more refined I believe, quieter, smoother and generally feels like a better quality bike in my opinion.
I'm not biased either way as I have never had a trials bike before, but I have had people say things like "that Beta is a nice tidy bike but I'd still have a gasgas anyday".
Having riden them both I tend to agree.
As for 80 or 125 I think there is very little difference as the 125 wont tear your arms off.