Bigger tires are almost always better.

Assuming they fit the bike and are of the same design, they will have less rolling resistance. Yes, they are a little heavier and less aerodynamic, but weight isn't your primary concern, and Aero doesn't mean much below 20 mph (it's effects increase exponentially). If you want more information google: bicycle quarterly tire pressure. They have inspired continental, michelin, and some german magazines to do testing that backs up the bigger is often better (especially when considering the same model of tire).

Bigger tires allow you to run lower pressure, give you more traction, and have fewer pinch flats (doh, I just got one of those last night!).

Bigger is not better when bigger means a stiff tires that have a lot of rolling resistance. But, in your case you are looking at the same tire, so that isn't a trade off.

P.S. I have ridden tires in both 1.5 and 2.0 size. Not much difference - the 2.0 tires may accelerate a little slower due to weight, but cruising speed is the same. I like the look of the bigger tires anyway. ;-)