definitely want the slick. I had a cheap mountain bike tire on one, similar tread as the bontrager tires on $300 LBS bikes. Loud as heck. Swapped to smooth, and could hear much better. Of course, the drivetrain was noisy, but that's a different problem.
Only thing I did notice, even with smooth tires in the highest gear you'll get with a 29er, the resistance was still real low, but I did have one of the cheap $70 trainer off amazon.
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