My preference is to under stock and I actually like the way the tank looks that way. I've always purposely under stocked. It also drives me nuts to see fish so crammed together that they are 'seemingly' having trouble navigating the water column. I've never moved past this point in some habitats that cry out for over crowding to reduce targeted aggression.
Also, over crowding 'tends' (or 'seems to me' at least) to have remedies that I prefer to avoid like no substrate, 50% WC weekly (or more), constant rearrangement of the habitat, seeking to remove partially grown pets, etc.
Under stocking's plus side is:
1) allows me to make mistakes or have temporary problems occur (e.g., power outages) with reduced chances of losing the fish I've worked years to grow. I have had power outages that lasted days and one for a full week. If my tanks had been over crowded it would have been disastrous.
2) allows me to more easily track individual fish eating habits and behaviors
3) gives me more flexibility in my habitat design
4) gives me more flexibility if I later need/want to change something in my design
5) gives me a better chance of growing out all my fish to the maximum size I want without removing some, adding filtration, changing tanks, etc.
6) gives me a chance to raise some reasonably personable animals that under excellent conditions can survive a long time
7) provides better opportunities for my fish to have much less stress and thus for me to have much less stress
Over stocking's plus side is:
1) I can keep adding fish to my tank and not worry about making the 'right choices' when I stock up the tank
2) I can start off with a lot more fish and maybe see some species that I will like
3) I can see some behaviors that I might not see (hyper aggression, breeding, etc.)
4) I can have a tank that looks more full and more active
I of course am not discounting the value of over stocking (anymore than shoaling) when those species warrant it. However, on a general level, and taking that into account, my preference remains to under stock.