I think probably less because you don't have to work as hard.

well .. depends . you can swim faster so if the distance is limited you plan to swim no. But if you swimming for say 30 minutes and distance is not a factor then yes because it is harder to move you legs with fins on. you have to work harder.

I think less because you do not have to work as hard with your legs!! But i am not sure because at the same time fins add a lot more weight on your body!!!

The only way to compare is to swim identical distances. RESEARCH SHOWS that using fins to supplement your swimming (short ones as long ones require too much effort) absolutely increases the number of large lower body muscles you use and, therefore, increases the number of calories used over a the same difference than without fins.

http://www.bukisa.com/articles/52854_how-to-lose-weight-swimming
or
http://www.forcefin.com/FF_PAGES/Truth_Swim2.html#fitness

Yes, the fins give your feet extra area of water to push with each stroke, requiring extra energy ...thus more calories. Fins will take you farther with each stroke, but swimming for 20 minutes with fins will burn more calories than 20 minutes without fins.

You could try that in one of our never ending exercise pools.