I live in NYC and have been a AA Platinum for 5 year. This year I reached Delta Gold and will be reaching Platinum by year end. I abandoned AA for Delta because of the favorable treatment given to this mid-tier elite level as far as upgrades go. AA requires you to use stickers and First is often already full. I have been upgraded on most Delta flights this year with no problems.

Now to the questions.
For the NY market, is CO better than DL or the reverse? CO matched my Gold and I am not sure if it's better to switch to CO and the Star Alliance.
Is CO or DL better with upgrades?
Who has better fares in NY? EWR is slightly more difficult to get to than LGA and JFK for DL but it's nice that CO is consolidated in one airport.
Is elite easier to maintain with CO or DL?
Are award tickets easier to book on CO? DL has watered down their miles and the Low, Mid or High ticket options are a joke.
Which Credit Card is better? Presidents Plus or Delta Reserve?