I haven't paid too much attention but I can tell you that TD's damage is not based on gear or offense level.
I use Greedo and Zam in my arena team.
Zam is G10 and Greedo is G8. Her offense is 100+ points above his and their detonators do identical damage when they don't crit.
I will have to pay more attention to when they do crit because Greedo's crit chance and crit damage are both higher than Zam's.
TD's damage is based on the target's max health and the amount of armor/defense.
EDIT:
After comparing their crit damage, it is also the same.
I've had several instances where one TD crits but the other doesn't so I'm not sure if crit chance affects that or if it is just purely luck.