As far as I know, DD 5.1 EX is the same as DD5.1 (i.e. standard dolby digital) but with a matrixed rear surround i.e. 6.1 but not true 6.1 like DTS-ES discrete.
I generally prefer DTS over DD5.1 as it's less compressed and just seems to give a better sound. Both are 5.1 methods from 2 different companies. DD5.1 is far more widely used though.
If you're comparing DD5.1 EX then like I say, it's got the extra rear channel so it comes down to personal preference.
In simple terms I think it means that they "calculate" it from surround signal. You've got 5 discrete channels and a separate LFE signal. It takes the left and right surround and plays around with adding bits and taking bits away (to put it very simply) to create the 6th channel. But with DTS-ES discrete you've got an actual discrete signal for the 6th channel rather than building one that isn't "meant" to be there.