NW was bought by DL; that is the merger that is happening, not CO.

From NYC you are going to get more non-stop flights on CO out of EWR than you will on DL/NW. As a gold (which is where you'll be with 50K miles) you'll not get too many free upgrades on CO. I think that NW will have more for you in that regard domestically.

Neither has free upgrades TATL/TPAC, and the rules on how they process their upgrades are rather different. On DL/NW you must have a ticket purchased in one of the top two or three fare buckets to use miles for an upgrade. On CO any fare is upgradeable but you have a co-pay of up to $500 each way as part of the upgrade costs, in addition to the miles. Personally I prefer the co-pay approach, but you might feel otherwise.