I am keen interested to know advantages of open Sesame over psychtoolbox. And what would researcher prefer when designing ERP paradigm: open Seasame or psychtoolbox?
I don't know much about open Sesame. But I have been using Psychtoolbox for quite some years now and as far I know it is extremely good and flexible. You can design an experiment with excellent timing control (given you have good enough monitors and CPU) and great flexibility. ERP paradigms and analysis can be easily done in MATLAB. EEGLAB is excellent for analysing the EEG data.
Spen Seasame is good for Python commands, as per my experience timing control is better than psychtoolbox. You may interface open seasame with Octave, MATLAB, Orange or Rapid-I.