The sprung Brooks seem more 'cush' over the long haul than the unsprung, and I'd agree with fixer, you hardly notice the spring factor. On my rides I've got a Conquest, B-17's and a sprung B-66. I think the sprung's are definetly more comfy and hardly less efficient.
And you will be the envy of fellow cyclists and pedestrians alike, a couple weeks ago I was stopped at a drawbridge waiting for it to come down, and listened to an older walking couple commenting on my bike and the Conquest. "With a seat like that, I'd like riding too" they were saying.