My bike is more aggressive than the Defy. I would say the seek is more comfortable at low speeds and just going to the store or whatever, especially with my bad back, however riding my drop bar bike is way more fun, and way better in the wind. I would expect the Defy to be more upright than my bike, but less than the seek.

I would lean toward the Defy on a 18 mile ride. The drop bars will give you more positions, and you'll probably drop weight and gain flexibility. You might really get into biking too, and then the road bike will be better for long joy rides.