David, I have a 2000 Tomos Sprint and a 2000 Derbi Revolution...I weigh about 195 pounds and the Tomos now that it is broken in which takes about 1200 miles, will run about 33-34 miles per hour indicated with tires properly inflated, no wind and on perfect level in a slight crouch....
The Derbi is not broken in all the way but seems to have a top speed of about 45 to 47 mph under perfect conditions (flat road, no wind, tires properly inflated and in a slight crouch. The Derbi has much nicer front forks than the Tomos which give a quieter ride and the fork geometry is more suited to 35 mph travel, affording a more stable feel. The variator belt transmission with more power than the Tomos gives better speed up hills although the Tomos will go up almost any paved hill at lower speeds.
If you like 50 cc machines, and don't mind travelling on slower roadways, they are both excellent machines. One last thought is on fuel economy...The Tomos will get 140 mpg flat out on when on long driving stretches with no stop and go operation. I have not had the Derbi on a similar trip yet but it will probably come in around 90 mpg but at a speed about 10 miles per hour faster...
Hope my comments help...FYI, Will