I read that the download versions are mp3s, whereas the physical products are not. And the mp3s have to be specifically modified to have the intended effect. Are there any differences in effects on a person from the download or the physical cd?
None, just make sure not to re-encode the MP3 in any way.
I offer CDs to serve those people who don't have or don't like mp3 players. The MP3 is built from the same files as the CDs are, at the same time. The only difference is that the mp3 is compressed using a special technique that allows it to retain the subliminal information. So it's really just a matter of whichever you prefer.