I have just built an automatic 2 ch dew heater, 4 outputs, and it's really easy to implement. Although, with all you want to do, perhaps the 328 would be more flexible. I had built most of it and I found the link below which gave me the idea of getting the dew point and do a small calculation based on your conditions, and the guy has done a great job of his dew heater.
As for the operation, it's easy to automatically do multiple sensor under dew strap readings and compare with your dew point. I use a DHT22 to get ambient temp and the humidity, then calculate dew point, compare with temp sensors under the dew straps by x degrees for your conditions. Then pwm to mosfets.
I wrote a small app for logging the data on the PC via USB and I'm sure it will come in handy for all sorts of things. Easy to get the data into Excel and do pretty graphs.
I also have a feature that if you startup the unit with switch 'A' on, it boots into a manual mode and bypasses the auto mode.
I'm using a 20 x 4 red chars on black background LCD in normal 4bit data mode, which is great to show lots of info. It's not the smallest box as it's the 'prototype', however I have a seperate small box that has the temp sensor inputs and the dew heater inputs which can sit on the scope. (very lightweight). I use midi connector for this - 2 pins for power and 3 for data. This small box and midi lead can be disconnected and the main unit used in stand alone manual mode without the sensors but still 2 ch with 4 outs.
Are you using mosfets for your output with pwm? I used the freetronics N-Drive mosfets and on a 10" dew strap, I can't feel them get warm at all. I'm sure though there are much better options though in the sea of fets.
This link is from another Ice In Space member with their dew heater.
http://www.iceinspace.com.au/63-597-0-0-1-0.html
Happy to help if you get stuck on the auto stuff.
Love your project and keen to see where it ends up.
Cheers,
Damien.