I have flown both airlines and aircraft. DL's 764 to LHR last year. Hands down select DL for your flights. Beyond the on-demand IFE at each seat and very comfortable seats and interior, the DL on board service is far superior to US as to food and beverage.....DL offers free wine or beer and will serve a main meal and additional snack prior to landing in both directions. DL is just better on overall delivery of product and service.
No doubt the A330 is a very nice airplane, but from a frequent customer perspective on both airlines - and I was a Chairman's Preferred with US for several years, DL has won my business.
Your domestic connections to/from LAS will be a Buy-On-Board service with both airlines but domestic USA service is pretty standard on everyone. Again, DL is the recommendation.
Good Luck with your trip plans, and I hope you enjoy your trip to the US.
watch what you want. you may get it.