My new custom Peter Mooney bike has 650 wheels and 26" wheels. I haven't noticed a difference in speed or acceleration relative to my older bike with 700s. I'm 5' 7" with a 32" inseam so could easily ride 700s except that my ideal top tube length and size 9 feet would have produced toe clip overlap with an ideal front end geometry. The handling of this bike came out so perfect that I wouldn't trade it for wider tire availability.

If you want versatility in tire width, then having both 650s (for 650x23 racing tires) and 26" (for 1.3" or wider touring/mtb tires) on the same frame is a good way to go. Switching wheels requires moving the brake pads up or down a couple millimeters. I was disappointed in the few options for 650 rims, since I built up my own on 28 hole hubs.