AV audio as in the white-red jacks?
digital output can gives you discrete 5.1 channel if you got the decoder that support it. while the red-white jacks can gives you 5.1 also if your decoder support pro-logic 2, but it is through sound processing from 2 channels, and not dedicated 6 channels.

red n white audio jack is only 2 channel stereo L/R in analog.
even with decoder with pro logic 2 processing it's still 2 channel stereo split into 5 channels.

as mentioned above, digital will gives you the best audio quality, clearer, crisper and sharper compared to analog. 

but it also depends on your audio source, garbage in garbage out theory applies here.  Also the type of cable you use for the digital audio.  Fiber optic cable will gives you the best performance.

My advice is... change to Digital, and you won't regret it

does your soundcard support audio-IN through fibre optic?
coz thats the only way if you want 6 channels sounds. btw, whats your sound card?

I was talking to some other forumer the other day who's also an AV enthusiast, and apparently HDMI > Digital > RCA(red/white) ... so since you're thinking of upgrading, might as well future-proof your system and go for HDMI!