If I've correctly matched the fares with the date (second half of August), then the DL fare is T, the AM fare according to matrix is N/V. OP, is you're a SkyMiles member, by my math you'll earn about 1/3 less redeemable miles and MQD on the AM fare versus Delta fare (but you'll save some $$). If you're a SkyMiles medallion, I expect you won't be able to enjoy the same benefits (upgrades, preferred seats) on an AM-marketed, DL-operated ticket as you would on a DL marketed and operated ticket (this applies to other partners, but I don't have direct experience on AM).

I assume you specifically want to go into EWR and will take a stop to do that, but just in case, there's a direct CUN-JFK (DL789) that is $240 on the date I think you're traveling. Departs 3:30 pm instead of 5:15 pm.