Could alternatively be an Enchantment, saying "Whenever a creature blocks or is blocked by a creature, the creature with higher CMC gets +1/+1 and gains first strike UEOT". (That'd be even more reminiscent of First Come, First Served than this version is.) Any thoughts on which is better?

I'd say the enchantment.

It's in the nature of CMCs that, usually, the higher CMC creature will be better anyway, so a one-shot effect won't turn that many combats around. An enchantment, OTOH, lets you build your deck around high-cost creatures that have lowish stats (presumably for some neat effect) and keep your back covered in combat.

That's a good point; maybe this won't really do enough usually to make it worth including.