At the end of the day, in my opinion, it all boils down to preference, convenience and discounts. Otherwise, there's no concrete reason to pick one over the other.

I prefer purchasing products from authorised resellers because I quite enjoy the traditional shopping experience, but I have purchased products off the Apple Store (app) and I've had nothing but fantastic experiences thus far.

If you're entitled to the RM400 discount on the Apple Online Store, then my suggestion would be to go for it!