David, the masters 35+ B is intended for "beginning" racers who are a little older and who want to race against other riders close to their own age and ability.
The masters 35+ B races last year were very competitive with some very fast racers. To realign the B class to the original intent, and to encourage faster riders to move up to the A's, the UCA membership voted to reclassify the B category as cat 4 and 5 only (cat 3 riders must now race the A category).
The hope is this change will make the racing a little more manageable for beginning masters age racers. I imagine next year, the speed in both fields will be roughly comparable (?), the fields in the cat 5s will likely be bigger. Not to say either is easy—bike racing is pretty hard at least compared to just riding your bike...
Good luck, and remember, if you don't like your first choice, you can always switch it up.
Jeff Clawson