18 miles one way is pretty burly. The Defy *may* be easier, and undoubtedly will be slightly faster. What will be better for you is tough to say... I love drop bars, and for such a long distance I would really want them. But I also love fat tires, and the Defy maxes out at 28s. For a guy your size I would also really want a bit more volume to the tires... Both bikes can take fenders and rear rack. I believe the seek can also take a front rack... I think with a few modifications to cockpit, the seek can be just as capable of a long distance commuter, and have a lot more options down the road.

If those were my only two options and I were my size (160 lbs) I might choose the Defy, slap a rack on it and swap the tires out for tougher 28s. But at your size my choice swings to the Seek. Throw a rack on it, and some bar ends to give more hand positions and you have a winner, I think.