Suzy wrote:
I am building a 70 cm Yagi. I want to make a balun out of a loop of coax
(yes, I know there are other methods but I prefer this one). My question is,
will rg58 do.

Yes, provided that you won't be exceeding its power rating.

Also, comparing like losses to like losses, should it be 50
ohm or 75 ohm and why?

Only the outside diameter is significant in determining the balun
operation, and the only real effect that will have is in determining the
number of turns and size you'll need to get effective balun operation.
You should use the same impedance of coax as you use for feeding your
Yagi, unless you intentionally want a mismatch for some reason.

Roy Lewallen, W7EL