Arc + sub vs 2 pieces of five, for listening to music, which is best?

You won’t be able to just use two Fives only for TV audio as you need a Sonos HT product to connect to the TV.

Having had the same/similar issue as you here, my chosen route was to add two Era 300s as rear surrounds to my Arc & Sub for TV viewing, but I like watching TV.  

For music, I put two Fives and a Sub (gen3) in the same room (out in front) for Music audio playback.

As you’re not too bothered about the TV viewing, I would maybe just use your Arc for TV only and get a pair of Fives and use your current Sub with those for music playback. 

I currently prefer the Fives set vertical at head height with the tweeters on the inside, but that’s a matter for yourself and your listening environment. I have the Sub in the middle, but in a corner, as shown in the attached ‘rough’ sketch. It works really well for our Living Room, but these things can depend on having power supplies nearby if, like me, you don’t like running cables around the room.
 

Yep, at least that’s the route I chose and it works well for our Living Room environment. The Fives are Sonos’ flagship speaker and a stereo pair (with a Sub) sound great for any type of music.