Yeah, not a huge difference. Personally, I prefer the 'Beos' color scheme, here in high contrast:
Here we have Midnight Meadow:
And last but not least, about Plasma performance. Some time ago, KNewsTicker was ported. Now there is a discussions going on about the release status on the KDE development mailinglist, and the KNewsTicker developer argued Plasma performance was bad - his applet used 15% on a 2 ghz Core2Duo, while just scrolling some text. Well - either he fixed the performance issue, he had a configuration problem, or someone else did something cool - but I can't reproduce it at all:
Further complaints about plasma performance (high cpu usage when moving plasmoids around) lead to Aaron testing the difference between KDesktop in KDE 3 and Plasma in KDE 4 - KDesktop used 3 times the cpu for moving an icon than plasma for moving a (transparant, svg, good looking) plasmoid... And to finish the 'performance sucks' argument off really well, Aaron mentioned there still is a lot of lowhanging fruit to fix, and Qt 4.4 is gonna speed things up a lot. Well, if Plasma performs as good as KDesktop with superkaramba, you won't hear me complaining (it does a LOT more). And it really looks it does - not just from Aaron's comments. Try KDesktop and Superkaramba with a few applets and be shocked (and that's not just superkaramba, all applet things in linux suck in terms of cpu usage - looks like Plasma is soon going to be the most sane implementation available).
BTW - see how cool KDE 4 looks ;-)
This is svn from a few hours ago, so the latest & greatest. And after playing with it for some minutes, I must say the main reason I don't switch fully to KDE 4 is because I installed it as a separate user, and I would have to migrate it to my current account or migrate my data to the other account ;-)