Eurostar question

No, this is strictly what currency you use to buy the tickets. Check the price for both, (or check it in EUR as well). Also check if the credit card you use to make the purchase has foreign transaction fees, often 3%. Some credit cards, CapitalOne comes to mind, have no foreign transaction fees. At the end of the day, we are probably talking about very small sums here. Maybe a couple of $'s.

Actually, checking for June 6, I get prices or 44 USD and 44 GBP for the same train (15:31), so the USD price is cheaper. Go figure.

Great Sam! Thank you.

See my edit.

A ticket is a ticket. The railway company does not care about your nationality, they don't ask that question (but the British and French authorities do, they check your passport before you board at St Pancras, including a French official who will stamp your passport of you are a US citizen).
You can pay in £, €, US$, the only difference is who does the currency conversion, Eurostar or your credit card company (Visa, Mastercard).

If you haven't done so already, inform your bank that you are about to do a "foreign transaction", or you risk getting it blocked as "suspicious". Whatever currency you pay in, it will still get booked in the UK (or France), and if your bank charges for foreign transactions, that charge will apply whatever currency you pay in.

Don't worry.. I have done it ( as a Canadian ) at least 4 or 5 times now. I have entered France or UK depending on what side I am starting at. they don't care.

As others have said, it doesn't matter.

Contrary to most other trains, security is tight. I would be at the station at least an hour early. The station is in a shopping mall. Last time we boarded in May the line to check in was outside the station, down the hallway in the shopping mall. Where and how you acquire the ticket is unimportant. In Paris you simply walk off the train. All of the immigration checks are done prior to boarding by the French. That is why the line is so long.