All things being equal there is probably a small rolling resistance advantage for the 700c wheel, very small. Moultons 18" wheel size was based on an aerodynamic advantage rather than a rolling resistance advantage.
Ofcourse gearing will be different but that just means you will use a lower gear to go the same speed with the same effort on the 700c.
Another small difference is the selection and availability of tires for your bike. The selection has improved in recent years but in general the availability of street tires for either style may not be the best. Fat cruiser and knobbie tires are easily available for 26" and skinny racing tires are easy to find for 700c but good width, high quality commuting tires will depend on you LBS. Thats why I usually order my commuting tires online.

Craig