North welsh sounds different to south welsh. There are differences in vocabulary and sometimes even grammar. I think it would be best to learn one at a time. North Wales has a higher % of people who speak Welsh, but south Wales has more people (however most of these people speak English with Welsh as a second language and it is not used much). I grew up in South Wales and have never needed to use Welsh.

I'm using Teach Yourself Welsh

You should learn both variants so that you can understand Welsh from anywhere (that's probably the reason why your book mixes them together), but when you're speaking or writing it's best to stick to one dialect at a time and not mix them. Which book are you using? Pob lwc efo'r astudio!