Hey; I agree with V8 Rumble on this issue. Waiting an extra month (nearly) can be a tough thing (we all know what its like, lol). So while the rest of us are playing GT5, you're left in the cold. Might as well have been delayed one more time for you.
That said, I will also say GT5
is totally worth a repurchase for you. Actually, if GT5 will be your main/only racer on the PS3, then it is even more profitable for you to get some extra cars, credits, and most importantly, practice. The only reason I see not putting in some extra Prologue time in is because you have GT4 to sustain your practice, and the credits, cars, and accomplishments are negligable in your opinion.
Also, because GT5 is a gift, it may not set you back too bad to trade in some of your games that you don't play anymore to reaquire Prologue. Its your call, of course, but in short answer to your question "Is it worth it?", then I say certainly.
Btw, why DID you get rid of Prologue???