[Superman]

My $0.02 (no instruments were used):

First, I assume the heat is all electric.....'strip' heater.'

During warm-up, all the strips are on all the time. During 'bake,' the strips turn off for at least part of the time. So....

Using the '250' option, one hour of strip-on-all-the-time is saved, compared to the 300 option. This is savings.

Now....strips come on and off during 'bake.' If the 'strip on' time for the 300 option (during baking, not warm-up) is LESS than one hour more than the 'strip on' time for the 250 option, then the 300 option uses less KVA/H. If the 'strip on' time for the 300 option is more than one hour more than the 250 option, then the 250 option uses less power.

I suppose it all depends on how well the oven retains heat.