My mate upgraded last week & we have to disagree. The new slim is just as clunky & he reckons if he was honest it's Noisier. Sound once loaded to Hd obviously but being smaller it gets hotter quicker so the fans spin up more.
Also the old fat one looks better & the dust doesn't stick to it, The slim matt one looks a bit crap to be honest.
Personally I would say buy the fat one & use the money to buy 2nd hand games, It's Brand new so warranty is not an issue, If 2nd hand i would go with the slim.