If all you do is stuff that doesn't require pro-grade colour management utilities [CMYK colourspace for printing/presssing colour images, higher than 32bit colours, stuff like that], then I'd say GIMP offers better performance/price ratio (being a free program). Which means that it is sufficient for a lot of stuff that you wouldn't think it can stretch to.
However, the interface design is inferior to Photoshop, and GimpShop version really doesn't help at all IMHO. GIMP is also a bit less stable on Windows than on Linux when handling images of extreme resolution; it seems prone to GLib memory errors if I boost the useable memory space over 1 GB.
Also, Photoshop offers better performance for some filters, at least compared to what GIMP did at the time I tested Photoshop CS2 briefly.
All in all, which is better for you depends on many factors. Do you need Photoshop? Probably not, aside from the more complex colour management situations. is Photoshop better than GIMP? Yes, in most areas, but GIMP is perfectly capable of doing most of the stuff that Photoshop does, it just does it a bit differently, maybe a bit slower and less stabler, but it does it. So if you don't have need for colour management gizmos, all you'll be paying for is the better interface and other basic stuff that photoshop maybe on some subjective scale does somewhat better than GIMP.
Is that worth paying the price of Photoshop for? That obviously depends on how much you need to pay for it. Hence I'd agree that if you can get Photoshop with extreme discount, it might be worth getting, but otherwise you'll need to weigh the performance/price ratio really carefully and preferably test both programs beforehand.