Is $400 the limit or a general target?
For $420 you can get a Giant Rincon
. It's a bit better than the 3900 you were looking at but comes with double walled rims and overall better component group. It also has disc brakes which shouldn't be the selling point, but they do look pretty cool without chewing up your rims the way V and cantilever brakes do. If you get lucky you might be able to score a leftover to get it back under 400.
For 419.99 you could upgrade to the Trek 4300
, the next step up from the 3900. The 4300 has a better rear deraillure than the 3900, stiffer rims, and longer travel in the front fork, 100mm vs 80mm. For a difference of $50 it's hard to see how you could go wrong with either the Rincon or the 4300.
For 349 the Norco Mountaineer
certainly is an interesting piece. It's got a rear V brake and a front disc. Kind of the opposite of what you want should rim wear be a concern. The rear rim will defiantly get chewed worse than the front will. It has double walled rims, 32 spoke count, a suspension fork with all of 50mm of travel, very poor quality deraillures...hmm...well the folks at Norco certainly made an interesting bag here. They cut some major corners to bring you double walled rims and one disc brake...I would give the mountaineer a pass if I were you.
The Trek 3900
is a nice bike by all means, you get a very nice component to cost ratio. The bikes geometry is not all that aggressive which will make for a comfortable ride. The Acera rear deraillure will work fine for commuting and light off road use. The Altus on my bicycle has traveled more than 700 miles without needing a single adjustment. Just remember to clean it and keep it properly lubed. Keep the chain clean too, your cassettes will thank you.
Being that you are a clyde (albeit a lighter one) your number once concern should be tough rims with a higher spoke count. Any frame should hold you just fine. I will say this, the learning curve with bicycles is very expensive. I have put as much money into upgrades as I have for the original purchase of my bike. If I were to put the money I put into upgrades plus the purchase cost of my 2.0 I could have purchased a much nicer bicycle than I currently have, oh well, live and learn then try to advise people not to repeat your mistakes. Therefor it is advisable to purchase the most bike that you can right up front.
It is my recommendation that you purchase the Giant Rincon if you can swing the extra $20. If not then defiantly go with the 3900 over the mountaineer providing you enjoy the way the 3900 rides.