Self study the 3 easier subjects first. Then send yourself up to Marochy and do the four harder ones.
I haven't done any other courses but Nathans course is ideal because you can re-sit any part of the course free of charge until you pass the subject which is great.
If you decide to self study the entire course then I would suggest doing them in the following order. Human Factors, Met, Air Law, Performance and Loading, Flight Planning, Navigation & Systems.
Doing Performance first will set you up well for Flight Planning.
Good luck..