Belegarth

What is the non-asthetical difference between metal and leather armor? The only thing I can see is an advantage in grappling, although it prevents you from grappling, to some extent.

In purely gameplay terms, you've nailed it. There is no in game advantage to wearing metal armor over leather, though there is a (very slight) disadvantage in that you're more limited in who you can initiate grapples with.

That said, metal armor tends to be heavier (though not always). We generally don't see plate very much. The two most common armor types are chain and leather. Leather is (generally) lighter, but chain breathes better. It really comes down to an aesthetic choice for most people.

You can also buy riveted chain for (relatively) dirt cheap online.