7.1 Setup, which sound codec to use?

Newb to the forum and A/V in general, using the Onkyo HT-S5300 http://www.us.onkyo.com/model.cfm?class=Systems&m=HT-S5300

When watching from HD (Bell 9241), I know it only outputs sounds in Dolby Digital 5.1 or PCM.

I was wondering what is the favorite codec to use in a 7.1 system in this case (with the Bell 9241), DD 5.1, DD Ex, Pro Logic IIx, or DTS Neo:6?

Right now I am usually using DD Ex on any HD channels that output DD 5.1, and DPLIIx for non-HD channels.

I do prefer to have all my speakers active, instead of just 5 speakers, but mostly what will generally sound better DD Ex or DPLIIx, I know in DDEx the rear speakers are in mono, but isn't it better because it's "digital"? Whereas DPLIIx is analogue?

Is there a big difference between these codecs? Also, for any Blu-Ray that is output in 5.1, should I stick with true 5.1 (not using my surround speakers), or go with DDex/DTS Matrix/DPLIIx when available?

Thanks in advance.