I didn't notice any difference between the two sizes. If there is it's 90% perceptual. But anyways I'd always take 700C over 27" simply because of the fact it's standardized with any equipment/bikes made in the last 25 years. I really enjoy nice bikes from the late '80s, more than any other period (haven't tried early '90s though!) and 27" simply won't fit in these bikes.