Ok, for me, I love Langakwi! It is quite, good beaches, very simple, more like "island life." There are tours that include feeding eagles from boats, bat caves, those little monkeys, a sealife farm where you can touch horseshoe crabs and see fish that spit (Mangrove Tour). I like the Holiday Villa Hotel. They have a fun Cultural Show every Saturday that kids and adults love. The pool is huge and clean, the restuarant has a great buffet, the staff is friendly, really good for the price. This is for the person that really wants that secluded, small town, island feel and who is not looking for others to entertain you every minute of the day.
Penang is very popular, but also much more commercial. Much more traffic, more built up. You would want to stay in Batu Ferringhi; they have a good night market. I like the Lone Pine Hotel there--more boutique, less expensive, big rooms, good restuarant. Also, there is a super fabulous private pool on the roof of the Bayview Beach Resort that you can rent for like RM120 an hour--totally worth it for like 2 or 3 hours in the middle of the day. You will be constantly entertained. And most likely broke by the end of your trip because of all the tourist boutiques.