Yes, 30min is stronger, for a number of reasons but the two principal ones are:
1) (I "know" this) the glue can "seep" deeper into the wood grain in 30 minutes than 6 minutes, giving a bond over a greater depth in the joint.
2) (I have "heard" this) the chemical reaction is cooler, slower, and the resulting chemical structures tend to be stronger because the chemical chans are longer.... (the person who told me this might have been talking from the wrong end....)

Regardless, it is generally acknowledged that 30min gives a better bond than 6 min. It is always recommended for more critical joints (Wing joining, firewalls, etc), wheras 6 min is mostly recommended for leass critical joints (servo trays, etc)..

gus