If you are set on flying from EWR then CO is probably going to be the carrier you use. If you're willing to consider JFK then DL comes into play as well. You can also mix and match the carriers to fly something like EWR-CO-BCN/MAD-DL-JFK on a single reward ticket if you want.

As for which carrier to put the miles in to make it happen, DL definitely has more bonuses on the transfers. CO last had a bonus offer in 2005 and does not expect to repeat the bonus in 2007 nor 2008. Both carriers can book the flights on the other carrier's planes, though you do have to call to make it happen. And ditto mia's advice of holding the reservation before making the mile transfer. Orphaning that many miles would suck.

As for which carrier to fly, CO is flying the BCN route on their 757 and the MAD run on a 767 (757 for the extra flight during the summer, but I doubt that will be in place next spring). DL flies it as a 767, if that matters to you. The CO 757s have AVOD throughout the plane (Y & J) and DL is working on fitting all their 767s with a similar product but I'm not sure the status of that effort.