Two arguments in favor of different MOX efficiency conventions:
1. Base the efficiency numbers on EU/t alone, as outlined by Omicron above. After all, many people will use AE to automate their reactors in everything except gathering uranium ore, and won't even see the system swap cells around. This would be the most useful to the typical player, I think. It's certainly easier to calculate.
2. Base the efficiency numbers on how much fuel actually gets burned up in the reactor, with respect to total EU gained from that material, assuming that players have access to a Thermal Centrifuge. As I don't know the various recipes myself, someone will need to fill me in here:
- How much U235 and U238 go into a regular nuclear fuel cell?
- How much U235, U238, and Pu does a spent regular fuel cell return when centrifuged?
- How much U235, U238, and Pu U238 go into a MOX fuel cell?
- How much U235, U238, and Pu does a spent MOX cell return when centrifuged?
The efficiency could thus be described along the lines of
TotalEnergyProducedPerCyclePerCell / (MaterialOut - MaterialIn)
which would actually return three numbers, one each for U235, U238, and Pu. Since these are all necessarily proportional to each other for any given fuel type, only one figure need be given, except in the case of hybrid reactors.