Staying 2 nights in December - the Park Hyatt is around $120 per night and the Rittenouse is around $170 per night. I have read that the Rittenhouse is one of the world's best hotels - is this correct? Or is the Park Hyatt just as good? Any comments appreciated.

Now that's DEFINITELY going too far.
The Rittenhouse is a nice hotel.
But please don't start rumors that it's one of the WORLD'S best.

It's only the 3rd best hotel in its own city.

It's not one of the top 15 hotels in the northeast IMO. Rooms are nice but a bit dated feeling. It's an older hotel and has a certain character you don't find in newer luxe cookie cutter hotels (many Ritz Carltons). But it's also not the spotless perfection you expect in a newer Four Seasons either.

Sure, it's a great place to stay in an excellent location. But don't get this 'world's best' tag started.

Fair enough - just regurgitating what I read on expedia, quickbook etc. I'm not saying its true though. How about a view on it compared with the Hyatt if you are so knowledgeable!

There's no question that the Rittenhouse is the better hotel, in line with the rates you were quoted.
The question for you is, is the extra cost worth it?
Both are older hotels with plenty of character. The Ritt is better maintained and has nicer furnishings. The PH is nice, but is more mainstream nice than elegant nice.
Why are you comparing these two, instead of Rittenhouse vs. Ritz, for example?

Thanks for those helpful comments - the comparison is mainly about price. The 4S and Ritz are both about $80 per night more than the Ritt - hence the comparison.

Check out the Westin and the Sofitel, great hotels, great locations.

I've stayed at the Rittenhouse before - a very nice hotel, (although not one of the world's best). To answer your question about the views - you either get one of the city or Rittenhouse Square - a nice city park. Either way, you're fine. Above messages are right about the feel of the hotel - however the bathrooms have that new feeling (shower & tub, tv, phone, etc). I haven't stayed at the Park Hyatt, so I cannot give a comparison, sorry.