Both Delo and Rotellla are very good HDEOs. With the advent of CK-4, Chevron elected to dual-rate their HDEO products to comply with the API CK-4/SN requirement of < 800 PPM of phosphorus whereas Shell did not. Rotella T4 is formulated with >/= 1000 PPM of phosphorus which makes it a logical choice for 6.7L Powerstroke owners who want to comply with Ford's warranty requirement...right, wrong, or indifferent.
Blackstone Labs published an article "The Oil Report July 2017" statistically validating that "Whatever differences exist from oil brand to oil brand, we don’t see a lot of difference in terms of wear". The choice of Delo or Rotella is really a user preference. Both oils will perform well if used within manufacturer's application and OCI guidelines.
The choice of conventional or synthetic is further a user preference. I realize that users who live in the northern US and Canada have a need for lower viscosity synthetic oil during the cold/winter months to facilitate diesel engine starting and warm-up. However, for those of us who live in more temperate climates, an appropriate API approved conventional 10W30 or 15w40 HDEO provides very good diesel engine protection.
So which is better: Delo or Rotella? There is no winner they are both very good HDEOs - both can be used with confidence.