For an incubator i would definately use a proportional thermostat... you will not get the temp fluxuations that you would with a ranco... and in an incubator thats important.

I just bought 4 of these today i hear they are better than Helix...? Plus they are not much more than Rancos
http://spyderrobotics.com/products/herpstat.html

Good luck!
Jon