I'm a little late but I'm finally getting around to "digitizing" my music collection. I can't decide on whether or not to use MP3 or AAC encoding. For the most part, I get the impression that AAC is ...

Since you already have everything lossless, it doesn't really make much difference. I would do them at 128 or 160, and then re-do any at a higher bitrate if you can hear artifacts on those tracks ...