Diafine is a compensating developer and with that capability comes certain benefits and restrictions. The good news is that you should get fine grain film development that doesn't requirement much temperature control in a very easy to use formula. However, that means you won't get the flexibility to change parameters (e.g. time or temperature) for certain desirable visuabl effects.
As for the differences between 35mm and 120, it could be due to the fact that 120 film has a great tonal range. Unfortunately, I'm not certain that is definitely the case.