Get the fork rather than the Fire Mountain. Having a nice fork like the Marz MX on your Giant will make for a better ride than another lower end bike, like the Kona.

Are disc's worth it? To me only if you have lots of $, or ride in wet and muddy conditions frequently. In the dry I find my Deore V's with MEC pads stop just as well as my buddies bikes with discs, although they have higher lever effort.

If you get the new fork... be careful it doen't get lifted like your last one! I use a cable lock to secure my front tire and fork (though arch and crown) to the rack.