Currently, BR11 normally departs no less than 30 minutes earlier than BR15. Currently EVA AIR uses 77A planes on BR11 while using the super old 77M planes for the BR15 flights. The average age of the 77A planes is roughly 5 years younger than the 77M planes.

If you are going to fly on business class, I would choose BR11, the 77A variant. The seats are the same reversed herringbone lie flat seats, but newer on the 77A planes in terms of age. The seats on the 77M planes are really old. For example, they're still using the old school style remote control and TV in the business class seats on the 77M planes. I don't think they have refurbished their business class seats on the 77M planes for at least 10+ years (not ever since they first introduced the Royal Laurel Class that we are seeing today).