Dan - it really depends upon what you are looking for. Kefalos on Kos is a really lovely resort, a laid back sort of place, good beaches, special just on outskirts of resort, although their in resort beach is fine. Great tavernas and plenty of good bars such as The Piano Lounge, Sydneys, B52, Opa Opa etc. - not not a really rowdy spot, more for couples I would say, but of ALL ages. September is a good time as it's not over busy and the weather would still be in low 80's. We have been as late as mid October and always had decent weather, the odd shower but nothing to write home about. If you are looking at Kos for nightlife then you could look at Kardemena, personally would not touch it with a barge poll, but that is my opinion and I am a fair bit older than you guys .
Wherever you end up - enjoy!