I’d keep my PC hard drive capacity at an optimum for my needs, rather than have too much capacity. I would also recommend using a NAS for backup and storage and this is where I would aim to have my high capacity storage (not without some level of RAID Protection).
Higher capacity storage = higher density = higher tolerances. Higher tolerances = higer risk of failure. I would therefore expect a 1Tb drive to have a shorter life than 0.5Tb as any surface defect would be more likely to span more tracks/greater relative surface area on the disc formatted at the higher density. The bigger drive is more likely to suffer an unrecoverable I/O error as a result.
These higher capacity units on average will probably last long enough for most people in normal domestic conditions(3 years or so), but I think the risk of premature failure is higher.
As user of laptops as a tool of my trade (large systems engineer), they were bounced around whilst travelling on the road, exposed to frequent swings in temperatures and generally hammered. I can tell you that the incidence of HDD failure was well below 3 years. This was with much smaller disc capacities tha 500Mb.
So whilst it sounds great having a terrabyte in one enclosure, the risk is that you could loose a lot more data. The best solution is lots of smaller discs in an array. Having a solid state disc in your laptop is the ideal, but these are expensive right now.
