I do not see any reason to like Arbiter over Arclight Vel or vice-versa. But if I had to choose...
Arbiter, purely because his overall design is clearly inspired by the Protoss architecture from Star Craft. It doesn't really make any sense why he'd be an Arbiter, but neither does Arclight.
You can probably make the argument that Arclight fits more so because there's an Arclight skin already for Varus...but what the heck is the "Arclight" skin-line even supposed to be? And how would you decide whether a champion is Arclight or Justicar? Aesthetically the two skin lines are completely identical.
This may be an unpopular opinion, but I think Riot should unify the two skin-lines, especially when the two of them are completely identical aesthetically. As for Vel...eh. It's just a name in the end, the Starcraft fan in me says Arbiter, but that's about as far as my opinion goes.
My only worry is that his super laser ultimate looks...smaller and weaker than his default skin's void laser. Less ravaging.