Course 1 vs Level 1 - which? Why? When? What?!

Just wondering whether to have a go at the northern challenges. I’ve finished course 3 (southern) and the the level 1 challenges as far as they go at the moment and am spending more time reading in Welsh than actually speaking. I notice that most of the written material for learners appears to be northern so thought it might do me some good. Does it matter if I end up speaking a mixture?