Actually that's what I have on my Cannondale R500 -- and WHAT a difference! (Plus the bike is nearly 15 pounds lighter). But you can tell the difference just by pushing each a couple feet. The Trek DS (with 700x38's) you have to PUSH. The Cannondale just rolls nearly effortlessly... There is one set of stairs with a bike ramp on one of the trails I ride and the difference is particularly noticeable. And, the Cannondale is a LOT faster than the Trek (which is weighed down with all kinds of stuff in addition to the wider tires)

But, I ride on limestone rails-to-trails -- and the wider 700x38's give a much more stability and safety -- especially this time of year when it is often wet and soft and covered in leaves and dead sticks and branches...

The Cannondale on it's 700x28's would be all over the place or the tires would sinking and slicing into the muck... Now that I have the Trek I only use the Cannondale on nice dry days on trails that are in good condition.

But, I am thinking of taking the Trek (which is my utility bike) down to 700x35's when the OEM tires show signs of wear...