The brand and model of tire will have far more effect on this than 3mm of tire size. A kevlar tire will be far better than a thin lightweight one regardless of the size.

If you are looking for something comfortable, run the widest tire that clears the frame and keep the pressure moderate. Honestly, on smooth trails you may not notice much either way. If your pavement has potholes then the wider tires will be appreciated. Try it out, bike tires don't cost that much.

I was suspect of them myself and thought there was no freaking way that a 32 was going to be as fast as the 23's I had on the bike. I wanted something more comfortable so I decided giving up some speed is ok to get that improvement. Well, after reading that same article on Compass Bicycle's site, I decided to give it a shot. I was very wrong. No loss in speed but good god these things are way more comfortable than 23's and with the added benefit I can run them down dirt/gravel roads without it beating the crap out of me. I'm sold now and will never go back to anything smaller.