I own a Bushmaster M-4, Cast Receiver CETME and a Stamped Receiver CETME. Very very different coice between the CETME and AR-15. I would not buy another stamped receiver CETME, but would jump at the chance to buy one of the original cast receiver ones again. I have had fewer problems with the cast gun than with my bushmaster. I experience jamming often using the busmaster and US GI magazines. I have found the Canadian Thermold magazines are the only ones that do not jam for me. My Cast CETME works with anything, CETME or G-3/HK-91.
As for shooting, if you like the power of the .308 and want a cheap gun go for the CETME. If you like smaller calibers, get the AR. It's all a matter of choice. You can do more with an AR. Accessories, different uppers, did I mention accessories? Accessorizing a CETME means putting a sling, bayonet and an aftermarket bipod affixed to the barrel. Unless you want to spend more money than the gun is worth buying an HK-91 collapsable stock and furniture set.
My real opinion, buy both! They are so different from each other you will have completely different shooting experiences.