been through similar here. Older boy is a small 13, about 5ft 3, and has for past year been on a 650B Cube Acid with 15 inch frame. Great Marz air fork, which is quite light. Think it retailed at around £800 but got with club discount for around £600. Full Deore groupset, xt rear mech
His younger brother, who is a large 9.5 (exactly 5 ft tall now) has just gone onto a Genesis Core 20, 650B, the 2015 one, with a 14 inch frame. It is a tad heavier than the Cube Acid and also slightly higher at front, as it has a 120mm fork – again air, a suntour xcr. We got this one for £450 in sales in January from LBS.
They both really benefit from decent groupsets, shimano hydro brakes and particularly air forks – both are set with around 50psi and light rebound, and they get full travel on them.
EDIT – The Genesis is still available from Sprockets (where I got it) at £450 – WITH AIR FORKS 🙂
here