I'd go with the larger one but that hard plastic case you linked is extremely bulky, I had a similar one on my iphone and I hated it. If you're going to jailbreak it, go into cydia and get something called "glovepod." It turns the volume buttons into next/previous and it makes the sleep button pause/play. This is at the lockscreen after you hit the home button to unlock it. You'll see what I mean when you play with it a bit.
The silicon gel case sucks if you constantly remove the ipod from your pants pocket. The rubbery feel sucks. But it can absorb some falls on hardwood, and tile floor. That screen protector is a rip off. iTouch and many phones use a special plastic that was created in the '50s for its screens, it hasn't been used commercially til now. Basically it suppose to be more resistant to scratches and not crack easily. I've had my iTouch for 2 years and it has one scratch.
If you throw your keys and iPod in the same pocket then you'll be more prone to scratches.
I wouldn't get a classic since it has a hard drive in it. Stick with a flash based memory model. You could always get an old school ipod mini on ebay for 20.00 and throw in a 16gb compactflash card for 40.00
It cost me 60 bucks and I have a converted mini with 16 gigs of space. Perfect for the mountain cuz it lasts all day and there is little to break on that thing.
I wouldn't bother with an iPod on the mountain. Get one of those $30-$45 el cheapo mp3 players which are uber-tiny and still hold like 4GB. Small enough to fit anywhere (you can even attach it to your goggle strap), and easy enough to tape up so that melted snow doesn't destroy it.
If your smart phone has enough battery life then you could try that, but 6 hours of mp3s + fighting for a cell signal on a mountain will probably drain it before your day is over.