As you’re probably aware, Cadw Swn teaches South Wales Welsh. That’s where I live, that’s the Welsh I speak, and that’s the accent I have.
Here as the main differences between North/South Welsh as I understand them:
The accent is very different, North Walians have a very strong nasal accent which some South Walians initially find hard to understand.
A small number of words are different throughout the country, lads, sweets, lasses and gates for instance. This is no big deal.
We abbreviate the present tense differently in the North and South; ‘I am’ would be ‘w i’ where I live and probably ‘Dw i’ in the North. ‘We are’ would be ‘Yn ni’ in the South and ‘Dyn ni’ (dun ni) in north Wales.
A few constructions are different; ‘Mae Ford gyda fi’ – i have a Ford (south) – Mae gen i Ford (north) Also North Walians tend to use ‘I did’ (Gwnes i) to say things like ‘I did shop’ – Gwnes i siopa. South Walians will probably say ‘I shopped’ – Siopiais i
As you can see, there are a few differences. Personally, I feel that Cadw Sŵn is such a useful course that the advantages outweigh the disadvantages and with a little practice you’ll soon get used to the differences. The choice, of course, is yours.