We are putting in pressure treated pine which will be painted. Here is a porch guide that gave very good info.

Here is a link that might be useful: Porch guide

Even though it is covered, I think you might consider spaced deck planks. On our current (old) house, our covered, screened porch has that type of floor and it is much easier to maintain and keep clean and dry. Ours is actually screened beneath the flooring as well, so every spring I do need to clean between the cracks, but finer dirt and all water goes down easily.

Painted floors, btw, are a pita to maintain inside, outside would be extra difficult I think. If you do paint, you might want to look into covering the paint, if latex, with a couple of coats of clear acrylic (not poly, too yellow)to extend its life.

We did our upper porch with PT pine. It is covered.
I used Cabot's Opaque stain in a gray color.
It is thick like paint. On the covered porch, the stain has held up well for 4 years.