Simulink: Solver Type and the implications of each

Hello,

I'm doing some stuff for work and I'm having a hard time determining what type of solver I should use. I'm dealing with a discrete PID loop and I'm wondering what the difference between the fixed step/variable step as well as the associated solver (ode, discrete, etc). How do these settings impact my simulations? If I simulate a discrete system with a variable step ode3 solver, will my results be inaccurate? Or for purely discrete systems, do I need to stick to fixed step, discrete solvers.

Any advice?