Ive been scouring the boards trying to find out what the difference was between DD+ 1.5 and the BD's 640kbps DD.

Can someone please explain the difference?

Lately I've been buying more HD DVDs than BDs because they say DD+ on the back.

Or is the difference not as noticeable, such as comparing a TrueHD track to a PCM?

The word from Dolby is that he sound quality of DD+ at 640kbps is equivalent to original DD, and that the differences lie in what the formats can do above that. If you're buying dual-format titles in red boxes, you're likely not gaining or losing anything (unless they're combo, in which case you got hit for $5).

Search around, there have been some great insider posts on this.

All of Warner's DD+ tracks on HD DVD are at 640kbps, the 640kbps DD on Blu-ray is of identical quality, so you can't go wrong with either one (unless one has lossless/uncompressed audio or IME and the other doesn't).

Paramount is the one you have to be more discriminating with. Their HD DVDs are DD+ at 1.5mbps whereas their Blu-ray discs are DD at 640kbps. I have no first-hand experience with these, but from what I've read the difference isn't that big, with the HD DVD being superiour by a relatively small margin (some reviewers didn't even know the audio tracks were different until they learned about the bitrates later). If you have both formats, though, you might as well buy Paramount's films on HD DVD, all other things being equal.