Personally I'd say use the single bed and get a rail! As for climbing out its bound to happen eventually and if he's not sleeping in his cot then its kinda the same isn't it. Seth will use his cot as a toddler bed until he is too short LOL! We were going to do this with Paris, but becuase my SIL gave us my neices old bed we moved her out of it, and also we were TTC and wanted to make sure the cot was long forgotten about before the new baby slept in it to ensure no jealousy issues. But as Seth will be our last we won't have to worry, and the longer I can go without spending more $$ on a bed the better LOL!

Goodluck!

*hugs*
Cailin