If I have to choose between those two, I'd probably go for Marshall, on the basis that I haven't played a single Fender amp which has suited my style, whereas I've been able to get more or less what I want from most Marshalls I've played through.
That's not to say Fender don't have some nice amps, I just haven't been lucky enough to find one I like.
However, I actually play (like quite a few around here) a Laney - just does everything I need. If money were no object, I could be tempted by any number of things - Cornell, Cornford, Mesa Boogie, Peavey JSX, Bad Cat, Bogner, Engl - the list goes on and on.... :)