off the top of my head, I would say that you pay for wireless technology in the z5450. Plus, it’s newer, thus it appears first.
I don’t think they replaced the z680… did they? I always thought the 5300 did that.
quality-wise, unless you’re a expertly trained audiophile, I think they are the same.
(my bro has the z5500… and personally, that’s all I’ll ever need. I have the 5300e, good enough for a college student who can’t blast stuff too loud ;))
edit: upon brief inspection of the product page, it seems that the 5500 indeed produces equivalent or better quality. definitely more maximum outage, slightly larger frequency range. I can’t seem to find the signal to noise ratio on the 5450, but I would imagine the 5500 to be equal or better.
it also seems that the added price for the 5450 is the enhanced/bonus functions. Better design (at least, I think it’s supposed to be better), the remote that controls more than just your speakers, and that nifty “digital soundtouch control center” are the main things the 5500 lacks.
These don’t affect sound quality, though… so it still holds that the 5500 gives better quality.
I still think that the difference will be negligible, but that’s because I can’t really tell good speakers from really good speakers
(audio-wise). Again, probably would only make a difference if you’re an expertly trained audiophile.
best way to find out is still to go into a store and give both a listen 
edit edit: was interested to know what happened to the z680. Comparing specs, it seems the 5500 replaced it.