I've been to both these restaurants, although not for a long while (about a year ago) and I would recommend Tia Pol if food is your priority. I've had many meals there, and they never disappoint. I've been to the back room for a NYE bash, and it's intimate, and almost bunker like. But with the candles and lighting, it was great.
AS for The Park, I've never been to the private room, but the space in general is pretty spectacular. But I've been there a few times and the food was ALWAYS disappointing. But I think people go there for the ambiance, and the food isn't very complicated, so it seems to do quite well.
But for my money, Tiapol for sure. Great tapas, great wines and Sangrias! Fava bean spread, pork belly (it was a special of the day) and their potatas bravas are just lovely.

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Tia Pol
205 10th Ave, New York, NY 10011