Translation:This computer has more disadvantages than advantages.
Both are possible, but I would indeed be more inclined to use „niż” here, as it just sounds better (to me) when you compare the different qualities of the same object. On the other hand:
- „Ten komputer ma więcej zalet od tamtego” – here „niż tamten” would be a secondary choice for me, „od” sounds better.
Nevertheless, I'm pretty sure you can't extend this to some general rule, mostly the two constructions can be used interchangeably.
Perhaps a correct answer could also be, "This computer has more disadvantages than benefits"