Not sure you are right. Assuming the chances of finding an item in 2 hours of TM is *24 the chances to find an item in 5 minutes of GR (and I believe this is true) then the chances are far better in TM.Nope you would be wrong. TM is a 2 hour job. I have had 5 drops in 24 hrs. GR is a 5 min job, I have done under 8hrs of it, and have gotten now 5 drops. Most drops I have had is from Treasure Hunt which has given me 8 drops, but it has taken 39 hrs. Grave Robber is one of the better dropping jobs from this, in hours spent, but not in number of jobs. So as hours spent basically is related to energy use, it is better to use GR for more drops. Look at your analyser and you will see.
Let's assume you do 2 hours a day of TM and 24*5 minutes of GR every day and ignore the energy spent completely. Item drops are directly relative to the motivation. After 2 hours of TM your motivation is 95%. After one hour (12 times) of GR your motivation is 40%. Where will you get better chances for item drops ? What will happen tomorrow when you start with 100% motivation for TM and 25% motivation for GR ?
Unless you use a large amount of buffs to increase motivation GR is a terrible job to do.
Personally I have done over 10 hours of GR now, 5 * 5 minutes a day for about two weeks, and got 2 item drops. I know I am fairly unlucky and you can't learn much from such low statistics but still ...