The spitting sound was something I edited out on a whim last year for a previous soundtrack attempt, and the edit seems to have found its way into this version also. I'm surprised you noticed!
The spitting was obviously a mistake--they must not have noticed that it was a part of the dialogue track until after the mix was complete. They had Mark Hamill redub that line for the mono version, and this error is not heard in that mix. Why it was never removed from the SE mixes I don't know--guess they forgot and didn't notice again. It had sort of bugged me after Darth Editous pointed it out a few years ago, so I deleted it by looping a section of background noise. Not having it in the film is no great loss; I'm certainly not going to produce a new version just to put it back in, lol. But yeah, I guess the oversight does make it slightly inauthentic for that brief moment, alas.
I've done some comparing of Dolby Prologic II and DTS Neo:6 as output by my receiver, and for the most part I'm hard pressed to hear any significant difference between them when not listening ultra-critically. The DTS version does have a greater degree of channel separation, though it comes at the expense of the sound field being slightly less 'stable'. But they both provide pleasing results with matrix-encoded Dolby Surround tracks, and it wouldn't be easy to tell which was being used in a blind test. For non-matrixed stereo, the results vary by the nature of the source material.