Ashura is an odd ability, because it strips Zoro of even more mobility. I don't think being in a permanent Ashura state would ever be something Zoro would want to be in.
Gear 5 can do anything it wants, it's his fastest, strongest, most durable, most range, most flexible mode with environmental manipulation at that.
Though Ashura does give Zoro an impenetrable defense, any attack that comes his way has to go through his blades, so Luffy might actually struggle with it. If Luffy had ranged attacks not using his own limbs than he could counter it no issues, but that's not the case.