Hmm... in my opinion, i feel langkawi is more relaxing than phuket. You can drive in Langkawi and do things at your own pace. Whereas in phuket, there are more things to see and do and i dont suppose one will dare to drive there. :x
It depends what you will be planning to do and how you intend to get there.
I personally prefer phuket and we brought our son (13 mths, then) for elephant riding, playing around the beach. There more things to do in phuket.
It also depends if you're comfortable in taking a plane up to phuket. If you feel that you're not that adventurous, then driving up to langkawi would be a better option.