You can pick up a used Rad fairly cheaply. You will find it a bit firmer, maybe a few points. Not a huge difference. It just depends on what you like. I like flex myself but not beyond a certain point.
With regard to spin, the PT280/630 MP was never intended to be a huge spin machine. It has a dense pattern, 18x20. (The OS is 18x19). However, depending on your strokes, your string, etc., you can certainly get very good spin if desired. Muster and Kuerten weren't exactly lacking for spin.
All else equal, in theory, you might get more spin out of the OS due to the string spacing. But, you will probably lose some control as it is an OS. If I hit with an OS, I want to put it down in about 5 minutes and go back to a MP or mid. But, that is strictly a matter of taste.