I'm a big Spartan fan, but my two pence is this. If there were equal amounts of soldiers (IE - 6000 Romans), I reckon that the fellas that were tasked with guarding the "back door" wouldn't have been caught short like the Phocians were (I think it was the Phocians, I'm not googing it or anything). If this had been legionaries or auxiliaries guarding the goat path, then I imagine they'd have put up really stiff resistance, resulting in a kind of mini-Thermopolae, which in turn would have sapped morale from the Persians. I'm not saying that they wouldn't have won in the end, but I reckon that the overall professionalism of the Roman Army would have paid dividends here. If there had been 6000 Spartiates, the war probably would have ended there and then, and history would be different. But there weren't and it wasn't.
But these are always fascinating discourses, I love them.
Cheers
Russ