There are far more 737 operators around than ones using ATRs, plus the 737 market is where the growth is.
I would guess an ATR rating *might* be cheaper, but the name of the game is getting a job, it's got to be the 737.
If you did get a job on an ATR, it's likely to be a relatively crummy one, whereas if you choose the 737, you have a fighting chance of it being a good one.
Either way it's a risky way to get further into debt.