Energy levels are just plain low, in both calibres so it's all about accuracy.
You may launch a pellet at 11.5ftlbs but by the time it travels 40yds your down to the 7-8 ftlb mark, so a .5 to 1ftlb difference means nothing really, fact is you have a greater chance of putting the shot where it needs to be in .177 as your range estimation is less critical, especially with lighter flatter flying pellets.