The advantages and disadvantages of 700c vs 650b-sized wheels have been long debated and will continue to be discussed for a long time. Riders who are looking for on- and off-road versatility will be best served by the lower weight and better obstacle rollover of 700c wheels. Those riding rougher terrain will find that the increased tyre size and volume of 650b wheels offer a significant advantage in grip and comfort.
Many gravel bikes will be able to accommodate both wheel sizes, giving riders who invest in a spare set of wheels the option of 'two bikes in one': a 700c wheelset set-up with fast-rolling tyres for the road and a 650b wheelset with large, knobbly tyres for more adventurous riding. However, we think manufacturers are moving away from 650b generally with more bikes being specced with 700c wheels now.