Also had both, kept the Stryker XL. The size difference for what I do with each was sort of a non factor. I like the 500d for durability over the xpac on the sling. The stryker has more attachment options, and while the dump pouch on the front of the HB is nice, its takes away some modularity. I felt I could keep things tighter to the frame with the Stryker XL as well. No doubt though, youre almost splitting hairs trying to decide between them. Both are very well designed packs.
And might not seem like a big deal to some, the Stryker XL has a pocket on the frame side of the sling for a bladder or other flat items while the HB is flat xpac. Yes, you can hang a bladder from the frame without a pocket on that side, I just prefer having that pocket there for my bladder/maps/etc. If you put the bladder in the pack with a camp bag on the shelf, depending on the length of your tube, might not make it all the way around.