Personally I will go for Singapore Airlines.

Singapore Airlines cabins (especially the A380s, but 744s as well) always give me a cleaner, nicer feel.

Service on SQ is always consistent and better (in my view), in-flight entertainment is better, food is better (again in my own view) too.

Malaysia Airlines has a 34" seat pitch as opposed to Singapore Airlines' 32". But somehow with the A380s I do not feel cramped, in fact I feel more spacious with Singapore Airlines!

Hope this helps.