Both wheel sizes have advantages and disadvantages. 650c wheels accelerate
and
climb faster, but they also decelerate faster. 700c wheels are more
comfortable
and are more readily available if you need a tube on the road.

The only people who should be really concerned about wheels sizes are
particularly short or tall people. 650c wheels work much better with shorter
people, especially on triathlon bikes where the geometry prevents the use of
700c wheels on smaller bikes. Tall riders should stick to 700c wheels.