Price, features, color, ...(?)
Street price of
the AT4040 is more than double the street price of the B1 ($270 v. $120).
B1:
10dB and 20db pad settings;
6 dB high pass settable at 75Hz and 150Hz;
2.5 lb.
silver
AT4040:
10 dB pad;
8 dB at 80Hz rolloff fiter.
3.2 lb.
black
But, really, I recommend if you are looking for a comparison of mics, go to amazon.com and look them up - all the specs, plus, usually, thoughtful reviews; then check harmony-central.com for more reviews.
You're asking for someone to do a lot of work for you, to compare to mics that are generally the same type but with different features and vastly different price points.
You might want to compare the B1 to the AT3035.