It depends what you're going to be using the computer for. The chips score nearly equal scores in SYSMark 2007 (edge to the i5750) but then i5 750 beats out the i5 650, often handily, in most other tests. Only when limited to a single threaded application does the i5 650 gain the upper hand. Unless you have a very specific application that is limited to 1 thread and that you will be running all the time, between the two I'd go i5 750.
You may find this to be helpful: http://www.anandtech.com/show/2972/the-rest-of-clarkdale-intel-s-pentium-g6950-core-i5-650-660-670-reviewed
It answers your question about rendering, showing the i5 650 to be faster only when limited to a single thread. So if you're rendering with a program that utilitzes multiple threads, then the 4 real cores of the i5 750 win out.