Interesting question.
The small oven will take less heating than the big one, but will take longer to warm up than the fan.
Also it will take longer to cook than the fan oven.
I would say if you are slow cooking something on a low heat for a long time (rice pudding, casserole as examples) then the small oven would be more economical. Or even for one or two jacket potatoes.
If you are reheating pies or cooking high temperature food stuffs for a shorter period, the fan oven would be best.
I'll be interested in others' answers.