The aluminum valve covers are also a little more rigid, so if you accidentally overtighten the nuts or bolts you're less likely to crush and distort it like steel. Also, lots of alloy valve covers have fins cast into them, which look real nice. The "technical" reason behind the fins is that they are supposed to dissipate heat more quickly, but I doubt the effect is quantifiable. Some of the nicest covers I've seen are with the cover body done in crinkle finish back paint, and the raised fins and lettering (if any) polished bare aluminum. Those are sweet, and I think Moss sells one like that for the TR's, it's says triumphtune on it or something. You could probably do that yourself with a bare aluminum cover if you've got some patience with the masking tape and an exacto knife and a good quality high heat crinkle finish paint, like what Eastwood sells.