Irv,
Thanks for the very insightful reply. All three 'bents that I have mentioned have 20" wheels. I'm not sure of the bikes gear ranging that you were speaking of. I'm not worried about setting any land/speed records thats for sure. Im usually anywhere from 14-18mph on my DF, as long as I'm not lumbering at 8mph I'll be happy, haha. The biggest worry with the tadpole I have is visibility while riding on the road.
Unfortunately I dont think I'm going to be able to ride the TT or Sun Trike before I buy. There are no TT dealers near me and the LBS' I have contacted nearby would have to order the Sun.