let me just say that i do not have a high elo. not at all. so don't bother reading this if you're not interested in what i have to say.
i used to main nasus, and i'm gonna have to say that gangplank is a lot more fun. i've built nasus in nearly every way possible, and from my experience i've noticed that anything other than melee dps for nasus hinders his full potential. tank/push and mage nasus suck hard. sure you can pull it off, but it aint the best way to play him imo. nasus' main points are his ability to farm very effectively and his ability to build up damage with ss, but you'll have to be able to consistently last hit with it (not that hard). he's probably the best farmer in the game as you can drop a spirit fire and you'll end up last hitting every minion almost every time. in team fights he's a great hybrid of dps and support, as dropping a spirit fire will accumulate armor pen and in turn allow every physical dps on your team, including you, to do a lot more damage. your slow is for getting that one last killing blow in, but remember that it has a limited range and only affects one champion, whereas others such as tryndamere have an aoe slow.
gangplank on the other hand i do not know much about, but from what i can tell he helps just as much, if not more, than nasus in team fights as his aoe ulti can hit any area of the map no matter where you are, not to mention that if they get hit by it they will be slowed. he does great DoT with grog soaked blade, allowing him to get kills that he normally wouldnt have, his parley attack does a great amount of damage over a long range, and he can heal himself.
i think that you'll have to try both champions to see which suits you best. no matter what people say, you may like one champion more than another so dont let anything others say discourage you from trying both.