If you think you will ever want to try race gas, get a 4 mill, play with compressions, want better cooling, want to be able to pull the head (maybe to check out your cylinders) without buying new gaskets, want some bling and maybe a little different power or more power get a noss head.
If you just want some more compression you can get it mill .030". If you want a different dome shape or enough compression to run race fuel, you'll have to pay for the domes to be cut too. And, if your motor goes you'll be buying a new head and paying for that work again. A set of domes is $60.
I wish I bought an aftermarket head a long time ago. Would have saved me a ton of money now.