AAC has different ratios against MP3, so thats good question to ask.
AAC has slightly better quality than MP3 in higher bitrates, and is superior to in lower bitrates.
Sample rate: You should keep sample ratio, as resampling looses some quality and up-sample wouldnt do any good.
Bitrate: If you like to have quality comparable to MP3, choose lower when using AAC.
For example, for 320kbps MP3 like, I would choose 192 to about 226.
This doesn apply to speech in which AAC has better compression ratio, hence same bitrate would yield higher quality.
For higher quality (not highest) movies generally rippers use 192kbps AAC. That would suit good even movies with lots of music and background sounds and effects. For general use and mediocre DVD films 128kbps would do the thing. Regular person should choose 128kbps, audiophile or movie lover may choose slightly more.