Pros Marshall:
everyone has one...
They have passed the test of time
they have a very identifiable sound
they are valves ridden (louder than solid state ones)

Cons Marshall
everyone has one...
they have a very identifiable sound
they are valves ridden (they need more care and sometimes you have to change the valves)

Pro Peavey
Dude, that logo looks cool!
Eddie Van Halen endorses it
Solid state distortion, good for heavy metal
Cheaper
Need less care (solid state)

Cons peavey
Dude, why don't you have a Marshall, like everyone else?
Solid state distortion (not good for blues or rock)

There we go, trust you ears too!
By the way, some people use both at the same time: Eric Johnson, SRV, Allan HOLDSWORTH, and they all are considered as tone gods... So if you 're rich buy both...