you will basically get the same treatment on either airline. priority boarding, security and baggage handling. you can use the lounges in either JFK or EWR or MAN if you are flying business class. nothing if in econ, unless you are a presidents club member or a plat or black amex card member.

if you are not flying business class and you do not buy the ticket on co.com and it is one of a lower fare class, you will not get 100% milage credit towards future elite status or segment points.

im not sure on either airlines AC rotation, but i believe CO uses a 767 (or at least used to). you will get personal entertainment at every seat in either class, where DL will have it in BC only. however if you're on the CO 752, you will only get it in BC.