[fai]

I've searched the forums and the criteria they mention don't really help me

http://imgur.com/zohrj

In a.), the c=c bond is formed in the least substituted α-carbon which makes sense. If the c=c bond were to form on the other α-carbon then the methyl group would somewhat hinder the nitrogen's bonding orbitals from lining up with the bonding orbitals of the c=c bond so the enamine c=c bond wouldn't form here.

However in b.), the c=c bond is formed on the more substituted α-carbon, why is this the case?

In clayden organic chemistry text, it also says that "due to thermodynamic control, the enamine formation is reversible so the less hindered enamine should predominate"

[Telamond]

Are you familiar with kinetic and thermodynamic control=

A more substituted double bond = more thermodynamically favored product. (In the case of b)

In the ring system (strain is a bigger issue here) (a), it's like you said, due to the stereoelectronic effects the most-substituted enamine won't happen due to kinetic control (it has a higher activation barrier for formation).

If a reaction is under thermodynamic control you usually want reversible reaction conditions so that even if the kinetic product is formed (low activation barrier, but not the most thermodynamically favoured product) it can go the reverse route, forming your starting compound again and then work its way back to forming the most thermodynamically favoured product.