The classic should be the most durable. At least I think the stainless steel should be tougher than the 18K gold.
Everyone appears to e saying the same thing, so I think you're right. However, this means the Sport version, which I gather is intended for the more active lifestyle, is less durable than the Classic.
In the natural order of things, I would have naturally assumed that the Sport would be more durable. Even Apple's site lists them, from left to right, as Classic, Sport and Edition, which seems to indicate that the left most one (Classic) is cheaper then the Sport, which is cheaper than the Edition.
Not long until we know for sure, of course, but it would still be interesting to know now.