Easy: whichever one fits you best. If the carbon one doesn't fit, it won't be comfortable to ride no matter what. Personally, I do not like aluminum bikes at all and won't even consider them, so I can't comment on the alu bike (except to say that an alu bike that fits is better than a carbon bike that doesn't).

If they don't fit, can you find some other models to try out? Don't disregard steel, either. There are some really nice steel bikes out there.