OK, so I'm the first to admit that I'm still working on my Private ticket... But here's a thought: aren't ATC instructions always going to be given in Knots?
I know that airport/airplane related information (such as weather reports, ATIS, METARs, TAFs, etc.) is pretty much always given in knots...
IMHO (again, I'm a relatively inexperienced flyer) it would be better to go with Knots and get used to working within that scale for everything, rather than converting back and forth in your head all the time while you attempt to fly your plane!
If you really need to know the MPH for your own curiosity, you can always roughly get the MPH by multiplying the Knots by 1.2. Going "backwards" (dividing by 1.2) from MPH to Knots is a lot harder to do in your head, I think.
Just my 2-cents (which is more than I have to spend on an airplane at the moment)...
--Noel