if it is 2 x 3 hours sessions, then i would go the 6 hours.

but if it is 1 x 6 hour session, then i would just go the 3.

does that make any sense?

my ds1 goes for 2 x 2hour sessions and it is great. he looks forward to it and asks when he is going again so i don't think one session a week would be enough.