On our return trip we will have to layover in KL ( 3pm ) and will depart following morning at 10am . IS it worhwhile going into Kl for the night or best to stay at a KL Airport hotel .

I would say yes depending how tired you are. KLIA has a train that take you into downtown KL in 30 minutes. Checkin to your hotel and head out for dinner somewhere. For the morning, if you leave your hotel at 7:30 you can take the same train (KLIA Express) back to the airport.

It really depends if you are up to making the effort or just wanna relax at the airport hotel.