The main difference is manual trim can be dialed in quicker than auto trim, that is the main advantage.
If u go into a steep dive u will notice if u let off your stick it will actaully try to lvl itself out for a few seconds, that is because your auto trim is still set for a few 100 miles an hour back.
Auto trim is designed to autocalculate your trim settings for the speed u are currently flying at. Hence u should be able to let off your stick and center it and u should fly perfectly strait.
The advantage if manual trim is it can dialed in faster than auto, but if u get use to auto trim then there really is not much difference.
The big key is if u end up in a sustained fight then u can switch to manual trim and set your elevator all the way up for a even harder turn.
also for everytime u are on and off the throttle, and speed changes, u will have to manage ailoron trim on the fly as well.
I belive its preference, if u want a more real feal then go with manual trim.
I use auto trim 99% of the time, but will manual trim to pull out of compression or for that extra little push on a turn.