Trying to get a summary statement on the seamless quality difference of LAME 3.90.3 and AAC from iTunes 4.7.x.

I am using an iPod, and yes I realize AAC consumes more CPU overhead, but I want to not consider that right now.

The Roberto comparison test focused on 128kbps mp3 and AAC, but neither of those are considered seamless from my readings. I have no way to ABX so I want to know which is the general concensus on a seamless MP3 and AAC file?

From what I've read, in 99% of the music, --alt-preset standard for LAME and 192kbps AAC would be considered seamless for their respective formats. Is this the consensus?

At the given seamless settings for LAME and AAC (whatever they may be), which format is going to yield smaller file sizes, and on average, by how much less size?

Normally, when an encoded sample can't be distinguished from the original it's called "transparent" (I suppose that is what you mean by "seamless"). Of course this varies across samples, people and equipments, but for the majority of people and samples I'd say the settings you mentioned should be transparent, probably even less than that, depending on the circumstances.

The size differences probably depend on your choice of music, but with APS averaging at about 200-210 kbps (in my experience), the space savings coming from the use of iTunes AAC @ 192 kbps would be probably 10% or less.