Aluminium is normally lighter (which is generally neither here nor there), lasts longer (won't warp), easier to clean (non-porous, so ink is easier to wipe off), and costs a little more.
From my supplier aluminium is about 15-20% more expensive than cedar in standard sizes (i.e. the kind of thing you'd use for t-shirt printing), but cheaper in very large screens (not relevant for most people on these forums).
Personally I prefer aluminium - the price difference isn't significant (especially for such a re-useable piece of equipment) and they're more pleasant to work with.
I use both though - there's nothing wrong with wooden screens (assuming they're a high quality wood and well joined). Next time you need a screen I'd recommend you try aluminium and decide for yourself - it's at least worth trying.
Screens are very much a matter of use what you've got though - they're not worth replacing until they actually physically require it. Most of the screens I use are wood, because that's what's there, but the new screens I buy will be aluminium.