Solely from reading up a little about each product, the Z-5500 seems like the much better choice. Firstly, they are THX certified which, even though it may seem like a marketing scheme, is something really hard for any speaker set to receive that's not largely priced. The bass on the Z-5500 seems like it should be able to punch a lot harder than the G51's bass solely based on the system specs provided by Logitech. I do recall listening to the G51's at a local store recently and finding myself unimpressed by their sound. I personally use the Z-2300 2.1 speakers and couldn't be happier with them (they are one of the few Logitech THX certified speakers). So yea, my vote goes to the Z-5500's for sure.