The moment you post here, I will tell you DIY is better.
Opening the casing technically won't void the warranty, but that is up to the service center to decide. It's better to pass to the service center to add hard disk.
Royfrosty got a better idea on warranty part and hope he can answer your query.
Regarding brand, it's all heng suay. No one brand is better than the other, other than the warranty service they offer.
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