It depends upon the voicing you like better.
I have an A and love it. The A's preamp lead circuit is closer to the IIC+ in design than the B is. I do love using the Lead channel of my Mark IV as a Rhythm channel and then using a clean boost to get out in front of it.
I have played Bs and can't say I didn't like them but I like my A more. I am not sure if it is because the A is the amp I am playing or because I just haven't played the B enough. The difference is very subtle but after playing one a lot you can tell. It would be hard to tell if someone else was playing because the settings could be different besides the fact that noone plays the same. Tone comes from your hands. So with that in mind, go play both. Determine what you like better. The best amp for you is the amp that makes you sound better. Just remember that the Mark IV is a sensitive little beast and that it takes a while to dial the little dude in because every littlest knob tweak or twist takes you somewhere else and that doesn't even get into the eq sliders' ability to really shape your sound.
I know people that like their Rectos for lead better but then again they play Rectos.
I heard the clips of Platypus' Stiletto and kind of like that lead tone as well. I think that is because it is more Marshally and I grew up with Marshalls.
