I am off to the US myself in a couple of months and even though I have an FAA PPL and the US Embassy has acknowledged my eligibility to enter the US on the visa waiver programme (I am a UK citizen) they have advised me to apply for a B-2 visa. They want a letter from the FTO I'm renting from too.
The alternative is to get the M1 through a US FTO which I think is a little cheaper