Recognise you're travelling business, but from an economy perspective, i have had no troubles with Delta so far. Did a short hop on the CRJ900 which was much more comfortable than DJs E190. Delta's trans-con con 757 is comfortable too, seat back video has free satellite tv, plus extras on demand for a fee. Complimentary soft drinks and bar snacks are included, although baggage is US$25/bag for domestic sectors that aren't part of an international ticket.
The T5 process is smooth for arrival and connecting on to US domestic legs. Plus you get SkyClub access (includes showers if you want to freshen up between flights). You wont get lounge access on Virgin America in LAX (plus you'll have to pay for it at JFK if you want to use it there). As noted above, this could be an important considerations when weighing up the two.
My experience with Delta's Economy product, on the ground and in the air, has been fine so far. I imagine it would be even better in First.