I have the HMG Expedition pack, essentially a taller version of the Porter. I used it on a 5 day trip in the Colorado Rockies over the summer, and the performance was excellent. Light, carries well, and very comfortable.
My take on the difference is that the Windrider is more of an ultralight pack, with a main capacity of 2400 cu in, and an additional 600 cu in between the front mesh packet and the side pockets. The Porter is a straight 3400 cu in, all of which is inside the pack. With the Windrider, you'd probably use the outside pockets to dry items, or maybe carry a tarp, but they would be exposed to the elements. With the Porter you can always add a front pocket or use the daisy chains for added capacity.
You can't go wrong with either.