I wouldn't say more aggressive, but a little more raw. The B is slightly more modern sounding. Honestly, there's more similarities between the revisions than there are differences... If you like one, you'll probably like the other as well. They all sound like Mark IV's.
They are not as aggressive as a Mark III, but to my ears they are much more pleasant sounding especially at higher volumes. The Mark III's can get fatiguing and harsh IMO and they sound a bit sterile in comparison. Also, the liquid lead tone is much more attainable with a IV, they play like a dream when it comes to leads.