Thinking of buying either and being worried about fuel consumption - you really need to look elsewhere....
Not about being worried, but if you're factoring things like maintenance costs in to your decision between the two as discussed as above then why wouldn't you also factor in this?
In my experiance the e46 and e92 are about the same, the e46 is not frugal at all especially if you like to rev it out, I am surprised at how much of a drop it like to drink TBH.