in the UK, from what I can see, many people ride on 650b because they use tyres bigger than 38c. In muddy clay, you need good clearance.
personally, I prefer riding on XC tyre (50-584 or 52-584). I tried G-One Bite and Ultrabite in "supposed to be" 45-622 (they are 42) and I found rubbish on muddy path, woodland and on tarmac, it was the worst... so my 700c have panaracer slicks which are ok, the Vittoria Barzo is the best tyre I rode so far (closely followed by Conti X King and Race King).
for me it is about the tyre width, I take whatever wheel size providing the frame can take 50-52mm tyres. I have my eyes on a Kona sutra LTD 52-622 or 60-584