Should I disable mobile data while I'm connected to WiFi? If WiFi is activated, the phone expends energy trying to find WiFi connections, even if I'm not using the internet. Is the same true for mobile data? Does the phone expend any energy trying to keep a good data connection even while it's connected to the internet via WiFi? If so, I'll set Llama to disable mobile data while WiFi is connected. Will that make a difference to my battery life?
When Wifi is activated, all data is pulled down via the Wifi. It is a mutually exclusive/flip-flop mechanism. That is to say, if Wifi is off, 3G is used. 

And yes, regardless of whether one or the other is activated, battery is being used likewise :) 

No discernible difference in terms of battery consumption.