ALWAYS DO THE HIGHEST LEVEL YOU CAN DO.
No, seriously. If you can't do it, level up your Medabots and Medals ASAP!!! The increase in stamina is linear, but the increase in rewards is exponential. If you can't do it, then cheese it with a Level 90/150 (bot/medal levels) "friend" bot.
Also, if you can't level up a Medabot anymore, go for the medal! 40 Levels of medal passives provide more stats than 40 Levels of medapart upgrades.
See here for the schedule of repeating dungeons