I haven’t exchanged currency in about a decade, but I think this is hard to answer even in general because the rates that banks/exchange agencies give vary quite a bit even when they’re right next door to each other.

I think it depends a lot on where you are in the US. If you’re in NYC you can probably find a bank with a decent exchange rate. I live in a mid-sized city and an exchange costs me about 7% plus a $5 per transaction fee, so it’s much cheaper for me to exchange in Europre. However just withdrawing from the ATM in local currency usually works out better than exchanging money,

It’s been a while but I’ve heard it’s less expensive once you get to Europe. It is supposed to also be less expensive to use your ATM over there. What I do is to make a large payment on my credit card and use it like a debit card. This is supposed to be a good way to get a fair exchange rate.

As long as your only using it at a point of sale and not as a ATM card that’s a good plan! Generally I exchange a small amount of cash before leaving and then hit the ATM when I arrive.
Cheers
Howie