I just had to make a very similar decision: I am going to Mauritius also in January with family - I chose the Westin (for most of the trip) as I thought:

- the hard product looks like it will be nice and has some have stated, the Meridien is a bit tired
- it has a club lounge which, I have confirmed with the hotel, has an outdoor area - quite important if you want to enjoy a drink in the evening
- there was C and P available
- I'm going in January - the resort opens (I think) on August 1st, so by then hopefully teething problems with service will be to some extent dealt with.

Concerning Langkawi - I love the island, but it's completely different to Mauritius - I would say that if you like lying on the beach and swimming, Mauritius wins hands down, for Jungle treks, boat tours and (in my opinion) local food, I prefer Langkawi.

My wife and I will leave the rest of the family and go to the St. R for two nights - booke C and P for 10,000 +180 USD - looks like a fantastic resort!