Having a Brompton, a Birdy with 18" wheels, and another Birdy with 20" wheels, i find that the bigger wheels are faster. All three bikes are fitted with Kojaks, the Birdies have similar gearing. The Brompton has the tallest gearing, but can't keep up with either Birdy.

If I am going to take a flight, the Brompton will go with me because it folds the smallest. For long rides and tours, I'll use the 20" Birdy.