T8 will gain you energy savings, better lumen maintenance and better light output in the long run. However, T8 ballasts are not reliable and will add to the maintenance cost of the building. This will be bad for you during the warranty period, but good for you if you will be the once to change the ballasts later on down the road.
I would sell the T8 upgrade over the T12 electronic upgrade, because I'm sure an electronic T12 ballast is no better than a T8 electronic ballast in terms of reliability.