Basicly, if your under 18 you want a 125. This is because the acu have age restrictions on bike capacities.
Gasgas - feel short, rev well but have there draw backs. The reward good maintainence with a long life, however poor maintainence will be punished by the poor build quality. My mate has recently bought a 2010, and in every ride for about 6 rides something has gone wrong.
Sherco- Feel very similar to the gasgas. Bit more tourqy, and slightly better build quality of the two. Rev well but still must be maintained.
Beta (my bike of choice)- I have a 2007 model and find it lovely. They feel much longer Imo than the other two and grip very well. They do require maintainence like any bike, however the bare minimum should get you by...
Mine revs very well, and IMO revs better, has more touqe, and top end than my mates 2007 gasgas. Change the oil reguraly and it will last you years.
Rev-3's have always been good bikes for beginers and experts.
Max