I'll chime-in once more~all great points. I've spent allot of years shooting 870s (police, express, wingmaster) as well as a fair amount of time with Grizzlies. With the $350 asking price on those Grizzlies now vs. the good deals you can get on 870 Expresses, I can't imagine why you'd choose a Grizzly unless the hunting component of shotgun ownership is an afterthought. (not assuming, I'm asking) The Grizzlies are robust guns, but no better than the 870 Express from my experiences. Personally, if you're trying to justify the 12.5" Grizzly purchase with the notion of adding a hunting barrel down the road~have at it! However, there are some great 2-barrel combos available with the 870 Expresses a few times a year, you may end-up spending less and have a Remington-branded gun when the dust settles.
Yes...I know the pros/cons of Expresses but I've had them work perfectly, and had others that needed 10 minutes of TLC to run perfectly. No biggie. My Grizzly needed more than that, including smoothing-out some roughly-stamped parts so I didn't cut my fingers when I stripped it down! lol I think there is a compromise in either case, but my vote (for a hunting gun) would be to bypass the 12.5" barrel idea.