Personally, I always ask myself how fast I filled my existing storage. Lets say it took you a few years to fill the 1TB you have -- it might not be worth it to buy the 2TB drive. However, if you filled it within a year and think you'll continue to fill it quickly then sure, buy the 2TB.
One word of warning -- be careful not to buy more storage than you can afford to backup. If you buy the 2TB drive, and you care at all about the data you place on it, then you'll need to make sure you also purchase enough storage to backup the contents of that drive.
I'm no expert, but if it were me I'd go for the 2TB and keep the 1TB as a backup. The 2TB's are fairly cheap. You can get one for around $96 as opposed to $58 for a 1TB. That's $48/TB for the 2 and $58/TB for the 1!
Many people thinks that 1 TB is too much, but if you already fill it up, I will suggest you to go ahead and get the 2 TB now! Because later on you will be loooking to get another, so better get the 2 TB at once :D I honestly Have 2 of them and are great!!!
Look at what YOU NEED. Did you fill up your first 1TB within the time span it takes for the average drive to fail? If so, you'll want the 2TB. If you won't fill up another 1TB for a while, get the 1TB, because you DON'T NEED an extra TB. Even if it's just forty for the extra TB, it is still an extra forty when you can be paying less.
When I consider buying a hard drive, my choice is guided first by how much I can afford. As the state of the art advances there is a changing optimal size per cost that is measured by MB/GB/TB per $. Calculate which size costs the most or least based on size versus price. Match that against what you can afford. Usually you will find that there is a size in between the larger and smaller capacities where you obtain the largest capacity for the lowest cost per unit of measure.
As a side note, prices were falling for years and lately prices of HDs are relatively higher than they were due to floods in SE Asia where they are primarily manufactured. Also, remember the future is with solid state drives. When they become competitive in price and are of greater capacity, they will be the drives to buy. They will not be vulnerable to hits and kicks (my sons do that while playing).
It is a matter of need and want, personally, provided you can afford it i would go with a 2tb drive if it's for low prerequisite items (music, movies, pictures), though i do still prefer a performance drive over a green drive whenever possible.
If this HDD will be used for gaming, then it becomes more of an issue, size is not as important as performance of the drive in gaming, if your question is about the different sizes of this specific brand of drive, then the 2tb is more futureproof and gives you more headroom, though i'd still keep the 1tb drive for the extra space(Or for putting specific files, i run steam and pc games on a 1 tb drive and everything else on a 3tb drive), you can have multiple HDD's in a pc, you do not need to get rid of one for the other unless you do not have enough power connectors/SATA ports.
In the end though, it's a matter of need, if you filled your 1tb up fast, another 1tb will fill just as fast, so keep that in mind.
If you've already filled up 1TB of storage then it's probably a good idea to go with the 2TB just because you seem to have a lot of stuff. Also I'm sure the price of going from the 1 to 2 is less than buying a 3rd 1TB.