Z5500 anyday....the other is an old and EOL product i think, and Creative is not very famous with their RMA periods taking months(source: Google)
Logitech is superb performing set- monstrous audio output but only with dedicated sound card analog output(aka Xonar DX and above ).Their optical output is lacking IMHO,but thats understandable from their position as a PC speaker set rather than HT stuff.They have one serious flaw though, the backlight of the control pod 'might' die out on u for no reasons at all, have seen a number of sets doing the same-some even coming live after several months,just as random as they died.
Bass is heavy, extremely boomy unless corrected for via good sound card software.... but after some personalization, it should provide some serious competition at the price point.Make sure u get from a dealer u are familiar with since that can help in a speedy RMA process in case u have to go for one, and make sure u get 2 year Manufacturer warranty.
Over and out.