I think it is like this:
If you are trying to level up as fast as possible, therefore care about xp/hr the most, standard synth may be the best.
- Easier: Can just keep hitting enter
- Faster: No thinking time, and faster to 'get throug the synth'

If you are trying to maximize xp per attempt (i.e., you have limited materials and need to squeeze the most out of them), then perhaps standard synth is not the best approach