Originally Posted by
wgscott
If I want my gravel bike with 700C wheels to be more off-road capable, it is suggested that I get a supplementary 650b wheel-set. This is the current trend.
If I want my mountain bike to be more off-road capable, it is suggested that I replace my 27.5" (650) wheel-set (and bike, if necessary) with 29" (700C). This is the current trend.
At least the first one makes some sense. The outside diameter of a 650b wheel + wide tire is approximately what a 700C + narrow tire is.
The two scenarios are not quite analogous.
In the first, the reason for going to 650b in order to run bigger tires on the same frame while not messing with the geo too much.
In the latter (at least as you are telling it) you are just switching for the wheel size, not to change the tire size. You also are talking about different frames.
A better example would be comparing 29ers and 27.5+. Those often fit in the same frame without screwing up the geometry. Some people choose 27.5+, others 29. One is not necessarily more off road capable than the other, they are just different.