Wasnt prologue just an early build of GT5. All of the content ended up in GT5 any way. Polyphony's output has hardly been prolific this gen and I dont think mentioning Prologue helps their case much.
Polyphony put out GT 5 Prologue, GT PSP, and GT 5 and sold 18.4 million copies.
Turn 10 released Forza 2,3 and 4 in one generation. Polyphony have managed to squeeze out one game that was so lacking on launch (no online matchmaking?!) they've been patching it for two years since. A high percentage of the car models in GT4 were even ported over from GT3. Not to mention the fact that reviews wise it has fared worse than its competitor.