To be honest there isn't going to be a terrible amount of difference between the two.
If it really comes down it, choose the one with the accent you prefer!
My wife, despite speaking very posh when speaking English - is desperate to have a Gogledd accent with her Welsh - and I think she wants to do some of the Northern lessons after finishing SSiW in an attempt to get that!
In reality, the terms/words are a bit interchangeable.
I have a couple of friends from Machynlleth (for a long time considered to be the North/South border in terms of language) and they mix everything up.
I've even heard them say things like "Dw i'n moyn (I want - generally southern) llefrith (milk - northern) efo (with - northern) fy coffi (my coffee)"
Having said that, I live in Aberystwyth - and we are a bit Northerly. So admittedly some Gogledd words have entered my vocabulary and always receive a bit of a "look" when I go back down South!