I prefer the warm. I've tested several and the cheap one from Vicks worked the best. By testing, I mean I let them run and measured the humidity in the air.
Also, Vicks warms the air.
It's so cheap that you can throw it away at the end of cold season and not feel guilty.
My pedi said that it didn't matter either way. By the time they breathe in the air, it will be 98.6 degrees. She suggested cool mist just because kids are more likely to burn themselves on the heating element in warm mist humidifiers.