Tray warmers are easier, but maybe not as accurate. The danger with immersion heaters is they need deep levels of liquid to ensure they stay immersed. I have them in a Colour print drum water bath and they are very effective but once set temperature can't be varied easily, however with two heaters and a separate thermostat they are very accurate.
It's your developer temperature that's more important, the toners are done by inspection in good room lighting or daylight so temperature is less critical.
Ian