is the z906 better than the z506?
sure, quite a decent step up actually and more output volume. they will still sound similar (logitech tends to have a bit more bloated bass that sounds boomy) but its not terrible.
i do agree that home theater speakers are more ideal for similar cost but that depends on the cost in your country. sometimes costs are very high and it will cost much more to use hifi/theater than it would the z906.
a warning about htib sets since i saw you looking at one (home theater in a box)
-not all support 5.1 input! even if they have hdmi or optical. you need to make very sure it does before purchasing.
-if your receiver goes, you might have trouble finding a receiver meant to handle the speakers as most are low...