The overall speed is always a factor, as it determines how quick you act. If you carry too much weight, the speed goes down. Depending on your strength, the two-hander may or may not bog you down. My recent dark elf character couldn't fight with a twohander without slowing down, so he had to settle for using it as a projectile weapon and then hack away with a longsword. He was mobbed and squashed even then, however. (darn goblins)The bottom line is - just like in real life, in DF, he that strikes the first blow, if it is hard enough, may not need another. The problem is, landing the blow may be difficult if you get beheaded while you swing.
I believe weapon quality also helps you attack accurately, so the scimitar will be best despite the lower damage. Anyway two handed swords are so cliche. Go for the scimitar. :)[ March 26, 2008: Message edited by: grendel ]
A falchion is very wide-bladed, like a machete.
(http://upload.wikimedia.org/wikipedia/commons/7/7c/Falchion.jpg) A scimitar is thinner, and longer, like this:
(http://upload.wikimedia.org/wikipedia/commons/7/79/Talwar_Hind%C3%BA_SXVII.jpg)