Your lo is 20 months, right? Mine is 17, and though he never slept in his crib well or in his toddler bed (which generally just uses the crib mattress) we just purchased a twin bed for him, and he is sleeping SO WELL!!!
It's WONDERFUL!
The decision is ultimately your's to make, but IMHO, if you're going to put them in a place that they can get out of anyways, I'd skip the todller bed. I will with the next one!