The extruded one is the earlier one, later they switched to the sheet metal one. The base is a solid piece of aluminum and it looks like the fins are glued on.

I’ve heard that the later one was better for cooling but I’ve never gone through the trouble to do any tests. I’ve used the original in a dual 1.25 overclocked to 1.33 ghz without overheating problems and I really pushed that one. Once I was curious to see how long it would take to render a particular 1080P video clip. It ran full speed for about 2 days before it finished.