I believe use the largest gears you can fit that are within your gearing range.
I run a 75 on my TC3. I did this because my car was light on the motor side (even after the battery location modification) so I wanted to utilize the weight I HAD to have in the car before adding weight.
the weight difference in using larger gears is SO minimal, its not even worth thinking about. Too much solder on your packs can make up for that.
I also agree about the mechanical effeciency: more teeth is more effecient because of a less steep involute interaction.
ALSO, using a larger spur gear gives a slightly wider range of FDR's to play with., because the difference between one tooth on a pinion has less effect than with a smaller spur.