What is the difference between a sampled time system and a discrete time system?
The first one is a purely discrete model. You need to select a discrete solver in the simulation options to accurately simulate this system. The second system however is actually continuous because of the holders, so you need a continuous time solver.

The preferred approach depends on what your system is. If you have a discrete model to work on, you should prefer the first one. If you have an actual physical system that you want to control with a microcontroller, you should prefer the second one.