Cpl Clott,
Not everyone agrees with Arc's analysis of the two gliders. I have flown and instructed on both types in both the Air Cadets and with BGA clubs and each has its own good points and bad points.
Rather than get into a pissing contest over this it would be best to just say the Vanguard (K21) has better rudder/aileron co-ordination in the front cockpit and the Viking (Grob 103 Acro) is much better in the rear cockpit.
That probably equates to the Vanguard being better for training but I never had any problems with students in either aircraft. The whole thing is academic anyway since Schleicher refused to have anything to do with supplying the 100 aircraft needed by the Air Cadets.
ACW