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.
This has something to do with Polish phonology. It's called final-obstruent devoicing. So [d] will indeed be pronounced as [t] at the end of words.