I've seen a few threads about people getting cracked stones on Hearthstones when driven hard, but within the manufacturer specs. I've also seen a bunch where people never have any issues with them at all in many years of service.
I'm in a rental property that had the Isle Royale already installed for several years when I got here. I've had it severely overfired(I'm talking nuclear orange here, didn't know any better), as have many tenants in the past, and the stove still stands strong as silent sentry in my dining room, no worse for wear.
I can't comment on the heat qualities of the Mansfield, but it very well may produce more heat with that extra 0.2 in the firebox. I do know it will hold the heat longer and output the heat much more evenly than any cast iron or steel stove can.
As an earlier poster mentioned, if it's two different dealers supplying the stove, and you feel better about one dealer over the other, then it might be an issue of choosing the better dealer.
Comparison:
Loading:
Isle Royale: Front or top loading
Mansfield: Front loading
Construction/aesthetics:
Isle Royale: Ornate Cast iron, symmetrical double doors, very solid construction. Isolated reports of Quadrafire's baffles and firebrick being low quality.
Mansfield: Square simplistic design, beautiful stone tiles, asymmetrical single door(asymmetrical door takes a LOT away from it in aesthetics for me, but makes gasketing MUCH easier. you may or may not care about the symmetry) Isolated reports of stones cracking on some hearthstone models.
Heat quality:
Isle Royale: Fast, powerful heat. Peaks quickly, drops off moderately
Mansfield: Slow, gentle heat. Peaks slowly, plateus for many hours followed by slow drop off
Blower:
Isle Royale: Available
Mansfield: Available
Pricing:
Isle Royale MSRP: $2,499.00(base) - $3,249.00(all options*)
Mansfield MSRP**: $3199.99(base) - $4149.97(all options*)
The price would be the biggest factor in my opinion. It's a pretty major difference. I did, however, see a dealer offering the Mansfield for 2499.99, which matches the base MSRP for the IR.
* note that all options on the IR include porcelain finish, Blower, Horizontal Vent Kit, Warming Shelves, Warming Shelf Brackets with Mitten Rods, Fire Screen, and Outside Air Kit. Options on Mansfield include porcelain finish, rear heat shield, blower, and outdoor air kit.
** I did NOT find the MSRP for the mansfield on hearthstone's website, but found it listed on the site that offered it for 2499.99 - 3328.97. The website did not seem very professional, and I have no idea of the reputation of this dealer. I won't post a link. A post on arboristsite.comdated 04-17-2006 quotes mansfield base MSRP at $2449.00, going up to $3216.00 with the most expensive porcelain finish and all accessories.