I swear I should have set this question in a FAQ or something...anyway
Well firstly, the one at the top is a kyosho mini-z derived RC car. The one at the bottom is based on the Canned-heat chassis made by Taiyo.
Mini-z cars are far more complex than the taiyo cars. They have working suspension, fully customisable chassis components, like the suspension springs, stabalisers, motors, gearing, body shell amongst others. The mini-z also contains a differential which makes it more stable in cornering. It also contains digitally proportioned steering. This is probably the main reason behind the high costs. This technology is similar to the 1/10 scale Tamiya-like cars used in competition racing. It allows absolute control of the car. This means depending on how much throttle or steering you input, the car will react accordingly. A slight touch of steering, means the car will only turn slightly. Full turn on the controller and the car turns sharply. On a taiyo chassis you can only tell the car to move a preset amount. You couldn't for example coast the car at walking pace into a corner, then floor the throttle to move away quickly out of the corner.
I've got two mini-z KR cars and the taiyo KITT, and I wouldn't recommend buying the mini-z cars. They are just too expensive and you'll be scared to damage it. Best idea, would be to get the spare KITT and KARR shells. Ok you won't be able to race them but they sure look nice in their display cases.
http://www.hlj.com is the best place to buy the shells. Far cheaper than ebay.
Hope that all makes sense.
Sarfraz