When I bought the bass originally I used coated DR Black Beauties or Roto Nexus which looked cool with the all black and ebony. But soon went through that stage.
For the last 3 or 4 years I have used EB Group 1V 40-95 on my ray and on my precision and fretless aria
Rounds have too much zing for me and soon change their tone after a couple of gigs. I guess it depends on what you are playing and how often you want to change your strings, but the advantage of flats is they get better with age and last a lot longer.
Flats are also kinder to my fingers especially my picking fingers so I get less blisters.
Lastly, Ernie's strings are reasonably priced compared to some others.