Kuala Lumpur is nice. Also means you travel with Malaysia airlines - one of the better ones. Nice city. A few sights to see if you're in the mood, nice markets to browse around, excellent food, and relatively cheap accommodation. I would say it's an ideal stopover.
You'll also gain a day if you fly west.
Hi agree with BQ67IE,, if you fly with Malaysian they give you free return flights from Kuala lumpur to islands like penang so you can do a city or a beach break or both,, you get the same on the way back