If you click on these 2 links (to the Strida website) you will find the speed difference between 16 and 18 inch wheels.
http://www.strida.nl/english/products.php?subpage=50
http://www.strida.nl/english/products.php?subpage=sx
I have a Dahon D3 Curve and a Specialized Crossroads comfort bike (about 9 years old). The SC is easier to pedal and travels faster than the DDC in my opinion.