I had been planning to book the 10 min TGV but tickets need to be booked in advance for the best price and so far the best price is $35 per person (for a 10 minute ride). So for 3 of us, private transfer is cheaper, and as others mention, much easier.
As for cash, I found out my local credit union can get foreign currency so we'll get some Euros before we leave home to cover transfers, tips and other things that we can't do with card. This will also save us time so we can get into the parks as soon as possible.