Yeah, different types make a huge difference in smoothness, speed and feel. I wouldn't recommend yaks, unless you put them in the front. The razor pro/ultra pro wheels are great plastic cores so they would be good if you're just beginning. IMO...
Reminds me a little of the MGP sig wheels....???
http://cgi.ebay.com.au/YAK-METAL-CORE-SET-fLEUR-FRONT-WHEEL-SOLID-REAR-P-/150461362800?cmd=ViewItem&pt=AU_Sport_Scooters_Accessories&hash=item2308323270
"Yak metal core set"
It's a flavour on the right. lol. I think some people think a metal...