The Diamond back is the better bike by a long shot but the gearing on the Schwinn is better. You are going to be spinning out on the DB as you get stronger. You can find deals on cranks on CL so I would defiantly go with the DB. My personal favorite crank gearing for a Utility type of bike is 26/36/48 with a 11/28 cassette. It will let you keep a fast pace on the flat but still has plenty of low to get you up hills with a load.

Now a word of warning that 65er touched on. Older MTB bikes like the ones you are looking at are closer to a touring bike geometry but newer MTB have longer top tubes to make climbing steep hills off road easier. A classic road bike the top tube is close to the same length as the seat tube. A MTB it is longer even on the ones you have posted. The newer bikes you may run out of seat post when you get the reach right so you are wise to stick with the older models. Your top tube length is going to be the more critical measurement for a comfortable riding position something that is a bit of a Oxymoron on a straight bar bike. 15 mile commute I sure would be looking at a touring bike.