The aftermarket mounts are much stiffer. This translates into more vibration and noise transmitted into the car. The benefits is it reduces driveline deflection by not letting the engine and transmission move around near as much.
I personally like them in a car that is meant to be driven with enthusiasm. But there is no benefit to doing in a car that you will only use to go stoplight to stoplight between the grocery store, home and work. I have used them in most of my cars, but be warned, do not do it if you think noise and vibration may be a nuisance to you. It will go up exponentially with a stiff aftermarket mount.