A tank with breeding groups will produce fry (if you later decide to sell or just have more fish), you'll also be able to exhibit fish's natural behavior (males showing, breeding, etc..), it's also easier to set up, without dealing with any added aggression, just get groups of less aggressive species and with the correct ratio of males to females you shouldn't see any psycho killer behavior.
With an all male tank you get a variety of color and species (as essentially you're stocking with 1 male of different specie). You also won't have to deal with any surviving fry. The tough part to an all male tank is that you'll need time and lots (maybe not alot as its different in every situation, but still as you're growing the fish out and can't vent them when they're small you're more than likely to end up with a few females in the bunch) trips to an lfs for returns or trade ins, to finally settle on a tank that is balanced and not crazy with males constantly fighting.
