Alu frames are nice but, I find, really hard on the body over long distances (probably because I'm close to being an old fart
). You do get very good spec for the money though.
Carbon on the other hand is far more comfortable to ride over long distances. It's corrosion resistant (far more than alu) and can be made just as stiff laterally as an alu frame while still being vertically compliant. Weight difference between the two will be minimal at around the grand level. The alu frame will be about as light as it can get whereas the carbon frame will be at the heavier end of the spectrum (as far as modern carbon frames are heavy).
The major drawback with carbon is its crash resistance, or lack of it. If you get a deep scratch in a carbon frame you can pretty much write it off. On an alu frame such a scratch will be purely cosmetic.
Speaking personally, I'd go for the carbon frame if it was my money because I don't crash very often.