i believe aluminium booms definately soften and fatigue over time....
i have carbon booms that are 5 years old and still stiff although as they are not continuous tail versions they are starting to show signs of wear. i am not a pro but i can pick the diffeence straight away between carbon and aluminium.
this is what fiberspar say..
www.fibersparsports.com/whycarbon.htmlthe stiffest boom i have ever used was the fiberspar carbon posigrip. i used it on a 6.4 kult. prior to that i had aluminium. going to carbon made the sail respond superbly. the entire rig felt locked in and predictable. a damn site easier to sail. however, i managed a break the boom when i botched a forward attempt. still unable to do them..... i found real benefits to a stiff boom for bump and jump and flat water blasting.
regardless of whether you get carbon or aluminium i think the following important factors when comparing booms.
a continuous tail piece and head piece. i have also found that in the larger sizes, (170+) booms, with wide tails they feel stiffer than the same booms with standard tails. (don't know why)
also look at the locking mechanism. the twist locks system fiberspar use is easy and quick but will wear out. go the twin pins if you can.
try and get a boom with an articulated head. saves the mast.