Rationalizing enamine stabilities
Conjugation effects are not very important in this case. 
The main reason is steric hindrance. If the more substituted alkene is formed, then both the methyl group and the nitrogen ring will be on the same plane, and repulsions will destabilize the molecule.

In the case of A, the methyl group is more free to move in a pseudo-chair conformation of the cyclohexene ring.

But there are exceptions, for example:



In the second case the more substituted alkene is formed, because is more stable and there is essentially no difference in regard to steric hindrance.