As a general rule of thumb, smaller wheels are stronger and more aerodynamic, but are more responsive, have more rolling resistance and are less comfortable. The drivetrain will also be slightly lower to the ground.
Gear ratios don't affect speed, it only means you need slightly different gearing.
The tri community went through a spasm of pushing 650 a few years ago, partly due to the aero advantages, but they've pretty much gone back to 700c.
In terms of performance, it's pretty much a wash.