Decimation: The scope will show only 1 point every X time steps. Whatever the rate of the input signal, with a decimation of 5, the scope will show you 1 point every 5 time steps.
Sample: You want a fixed rate. one point every 0.001sec or 1 point every 0.2 sec. In that case, even if the solver would not need to take a step at that time, it will take one to generate that point.
As you can guess, the difference is especially important if you use a variable sample time solver.
