I’ve carried both my B25 and my B30 walking for miles in NYC, Paris and SF. While obviously the B30 is marginally heavier than the B25 empty, as
@WhiteBus points out, it can accommodate so much more stuff so it inevitably ends up being quite a bit heavier. I can however, slide it up to my elbow and carry in a different way. I am headed to Paris for just over a month in April/May and will bring both but will probably carry the B25 more often as it will be lighter.
However, I have no interest in another B25 and would take the right B30 in a heartbeat