It partly depends on what type of riding you'll be doing--racing, short or long recreational rides, some of each, etc. How much do you want to spend? How tall are you? How heavy a bike will you tolerate? A tall all-aluminium bike might ride more smoothly than a short one. Carbon seat stays will smooth the ride somewhat. A lot of folks think steel's the smoothest for long rides, but it tends to be heavier. Lots to consider!

Cheers.