If you'll ride it, get a nice bike. There are some good value carbon bikes out there (yt, fezzari) and you can also get a nice but cheap carbon bike from last year's demos. While I ride carbon everything, I bought my wife's bike in aluminium, and I think that was the right call for her as she doesn't ride much. Her bike isn't really that much slower, but it just feels less playful and it feels kind of clunky. If I only rode once a month I'd get aluminium.