The basics...
Maxima Pros/Altima Cons:
Max had years to develop chassis and work out the bugs; Max currently has massive aftermarket support (well, for a Nissan); Max is relatively elite for a mid-size sedan.
Altima Pros/Maxima Cons:
Altima has lower pricetag; Altima has fresh aesthetics and engineering.
Anything else to add?
I went with the Altima--nearly the same stuff for a more affordable price (but I know how the Maxima feels to be of a higher quality).