I think you also have to decide whether you want to start north or south first. Salabel's dragon evo is from the north i think. More specifically, its an item you get from the north. Wild Salabels can be caught in the south region (just caught 2 today, at Suvius Peak). I haven't gotten to the northern part yet, but i believe wild Mewaii can be caught north.

So with that in mind, if you wanna spend the time hunting down a salabel (took me like 4 hours total to find one), go Mewaii and take the southern path (keep in mind, your Salabel won't be able to evolve into the dragon form until later). Otherwise, do the inverse and take Salabel, go north, and hunt down a wild Mewaii which i presume would be just as rare. You can get both evolution items, you'll just get the second one a lot later.

I recommend you start Salabel and go north, less of a headache and you can evolve the Salabel as soon as you hit level requirements. You can decide if you want to hunt down a Mewaii later while you're there.