There are a few big differences I notice between PL II and DD 5.1 (my stereo system is 5.1 and ran me about $1300 or so... it's been awhile since I set it up).
The subwoofer is a pretty big difference. 5.1 does a much better job of utilizing the sub for really deep sounds.
Positional sound is a HUGE difference. In PL II, it's a rough estimation of where the sound is originating. In 5.1, you know exactly where the sound is coming from and it helps a lot when playing epic-style games with massive battles going on around you (CoD4, Halo, etc.).
Overall, sound clarity is quite a bit nicer in 5.1 but that's likely due to the optical cable I use for 5.1 vs. the RCA jacks in PL II. It still annoys the shit out of me that Nintendo didn't include a $2 optical port in the Wii for those of us who actually spend a little money on our sound systems.
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