I agree with TarHeelsinNJ and given your choices, Helmsley or Essex might be best. However, nytraveler makes some good points and the UWS does feel like a different world than midtown. It's certainly not inconvenient to anything you want to do - in fact at On the Ave you'd be right down the street from the Museum of Natural History and a nice stretch of Central Park.

As for the hotels TarHeelsinNJ mentioned, the Hudson is a cool "trendy" place in a good location but has very small rooms. Park Central is nice and comfortable with much bigger rooms and an even more convenient location - right on 7th very close to the park and subways. That would also be a good choice for you and I'm sure it would be cheaper than Helmsley or Essex.