I have ridden this route a few times, I live in Royston, just before Courtenay. The route is relatively flat as you follow the coast.
There is a more direct route if you stay on Highway 19, which is the 4 lane highway. That has wide shoulders, a few more hills, but there are no stores or gas stations.
So in brief 19a has more intersections and chances of getting creamed, but the shoulders are normally pretty good.
19 is the big highway with traffic going fast next to you but with very wide shoulders and limited intersections.
Hope this helps.

Paul.