aluminium frames generally don't last as long as steel, which is ok if the alu frame ain't expensive. I'm reluctant to pay more than $1000 AUS for aluminium.

If you buy aluminium, you increase the chances of the bike being uncomfortable, but a lot can be done to improve the ride with the right carbon fork.

I cracked 2 alu frames in under 2 years, but all 5 of my steel frames are still going strong (they're not all built up at the moment )