I haven't flown DL TPAC but I've flown them ATL-FCO and JFK-ATH, both slightly shorter than this flight.
I can't imagine this flight would be much different than those two. I would also be surprised if the food differs on the 767 vs a 777/A330. To me, I would expect roughly the same experience in this flight and aircraft as I would a TATL flight on Delta.
Now, the true difference is going to be between DL and MU. I've flown them JFK-PVG. They aren't bad. They're no Delta but they aren't horrid. The FA's, in my experience, struggle with English. The food is edible, but their western food leaves a lot to be desired. The IFE is, Chinese. Most western movies or shows was edited. You won't find as many current/new movies as you would on Delta. However, from JFK, I've found so many killer deals on them I can't say no. Think $450 rt JFK-South East Asia, 18K MQM's, 10K RDM's, $1K+ MQD's. That makes it worth it for me.
If price is equal, are MQM's and MQD's important to you? You might get more on MU than DL.
Final thought, MU doesn't fly into Seattle. They do into LAX, SFO, and YVR on the West Coast. Could the MU flight be a codeshare on DL to China? Or is the MU flight a double connect, through a US gateway and PVG?