The difference between road and MTB gearing isn't just how high or low the gears are,but also the range. Road gearing is usually pretty narrow to maintain a more constant cadence on long rides. MTB gearing is usually broader to allow for constantly changing terrain. The Fairfax's biggest gear is 52/12(big ring up front,8th speed out back) which according to Sheldon's gear calculator(http://sheldonbrown.com/gears/) is 116 Gear inches;the Coda's 48/11 top combo gives 116.8 gear inches,so the Coda actually has a taller top gear than the Fairfax. In hilly terrain,you're going to be mashing up steep hills and playing your shifters like castinettes with the Fairfax while the Coda will allow you to spin easier and shift less. Of course,you can always change the gearing on the Fairfax,I'm just compairing the stock bikes.

Carbon fork durability is pretty much a non-issue on street bikes unless you ride like an animal or don't take care of your gear. Don't ride down stairs and lock it where it won't get knocked about by foot traffic/bad parkers and you'll be fine.

For the adjustable stem,two words: blue Loctite. Problem solved. From the pic on Marin's site I can tell you that it sweeps up/back and down/forward. Can't tell if it can be raised lowered without moving the spacers around. You can always do that;most stock bikes are set up with a stack of spacers slid down the steerer tube with the stem above them and the top cap on the top. You could change the height of the stem by pulling spacers out from underneath and putting them on top.

As for the weight difference,there's alot of factors besides weight that come into play for hill climbing. Gearing,wheel weight,tire weight/pressure/tread,riding position...and the biggest thing is how strong your legs are. When I first started commuting,it was on a Trek 7200 and the hills kicked my azz royally. I gradually built myself up to where I could ride into/home from work without having to get off and push. Now my daily commuter is a 32lbs touring bike,and the hills aren't a big deal anymore.