In addition to what everyone has already stated, I love Brooks becasue of the saddle-bag eyelets! Brooks & Carradice, a match made in heaven.
I must say that my b17 broke in much faster than the swallow, and it's more comfortable for me - probably because it's wider than the swallow. But I still do 100 miles on the swallow without issue.
And weight-wise, you can always compensate for the few extra hundred grams by shedding some weight off the 'engine' if you know what i mean. =]