No matter what you pay for one, it'll seem like a bargain when you see daylight and your friends' smiling faces! Hopefully that day will never happen... I'm not familiar with the particular models but recommend you get the best, easiest to operate that you can and don't worry about a $30 (15%) difference, especially considering you're getting a screaming deal with the pro form.
I've got an M1 as my first, and loan that to less experienced folks as the occasion requires, while I wear a Pieps myself that's not as easy to use but still way better than what we had back in the olden days.
I'd avoid picking up a used one - "Wow, that thing hit the concrete pretty hard but still works - I think its time to put this one on Craig's List..."
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse