they will certainly be comparable. while i've not heard the z506 vs the t6300 on paper i have heard the 2.1 creative set vs the 2.1 logitech set and while there are slight differences they both arent bad for the money. the 5.1 sets should be similar in nature. i dont think you could go wrong with either.
with that said,
keep in mind that the 6300 does not have a horizontal center channel, instead its one speaker just like the rest of the rears. the z506 does have a horizontal center channel. you may get some more impact out of the center on the logitech and its not going to be in the way as much if you have just one monitor (since its horizontal).
i like that the logitechs have two drivers instead of a single one as well.
on paper...