It really depends what you want to get out of it and what you like, I love the tropheus don't get me wrong, but they are expensive, slow growing and can just all turn over and die. In a 720L tank to actually do it justice you would want to buy at least 30-40 tropheus otherwise you would have an empty tank even once they grew up, and if you wanted to breed up more then you would have to be patient enough to do so. The haps are cheap extremely hardy and easy to source breed etc, I know most people who walk into my fish room comment on how awesome the big male giraffe is but not many notice the tropheus although I prefer the ikola more..
Cost depends on what you want, males or bigger fish go for more but smaller fish to grow out are pretty cheap you could grow them out see what you like and thin out females as you go, things like dolphins/yellows look good in a group of males/females so could probably be left alone.