I think in the North they saw “medru” for “can” and in the South “gallu” (with that lovely hissing “l” that makes Welsh so unique). But all my speaking partners who are doing the Northern course understand “gallu” very well and don’t seem to have any problem with it:)
I think that what you should study depends on where you live:) But if you don’t live in Cymru, you might just follow your heart and choose the dialect that sounds prettier to you. I chose the Southern one because I want to visit Carmarthen, and it’s in the South.
Anyway, there are links to some topics with people debating which course, Northern or Southern, to do, here in this thread. Maybe it will be useful.