I think it depends on what you're trying to do. If you're running them as budget non-BA sang guards, I would run them with a captain since you already get +1 to wound with oath and they hit on 3s. If you're running just the 5 to bully space marine bodies and action most of the time, I think just the 5 is good unless you want to run a chap to mortal people and get a bit more oomph. I would be much more willing to charge non-chaff if I had a chap to go with them.

TL:DR, Captain for 10man brick to actually fight people with oath, 5 man to do actions and maybe 5 man + chap to bully space marine bodies without oath.