Here was my thinking when going for the 2TB drive instead of the 1TB. And I'll admit it's not all based on numbers. And these ideas may not appeal to you.
- I like performance, and since I invested in SSD for speed, why not get the best HDD as well?
- I can't predict how long a 1TB drive will last me, but I know 2TBs will last longer!
- If I need the additional storage capacity above what a 1TB can support, I have it all ready for me, sitting inside my iMac.
- If I don't need the additional capacity, I know I won't have to buy a defrag program like iDefrag and that saves me money (and the effort of running the program), and
- I want to avoid further splitting my data. It's enough of an effort to make decisions to place files on SSD vs internal HDD, without adding an external HDD choice as well. Storage management is such a non-productive task!
- I can't avoid having external drives (say for backup), but that doesn't make me like them. Even with the promises of Thunderbolt. It's just another bit of complexity and opportunity for tripping over cables and corrupting data.
The performance difference is hard to assess because it depends on how much accessing you do to the HDD. When you read manufacturer drive specs, they tell you seek time, minimum, average and maximum, but in reality, seeks are shorter than the average due to clustering of accesses.
That said, if you buy a 2TB drive and "use it as you would a 1TB drive" ***, then:-
The seek times will come down by very close to 50%. Average*** seek time according to the spec is probably about 9ms, and that will come done to about 4.5 ms. But you'll save a bit less than this difference.
In reality, you might be looking at seeing your 2TB HDD perform about 15% to 30% faster than a 1TB drive. At times it will be more, at others, less.
What this translates into as far as your experience is concerned is hard to say, but you know it doesn't get any better than this!
*** Notes
"Average" seek distance is 1/3 of the maximum.
By "use it as you would a 1TB drive", I mean never fill it beyond about 80% (800 GBs). I know others will say you can push the envelope further, but no matter how hard you push it, you won't get over 1,000GBs

And that means that your 2TB drive will have more than 50% of its capacity unused.
PS Don't feel bad about underutilizing storage capacity that you paid for unless you are prepared to feel equally bad about underutilizing a drive's ability to deliver data. You paid for that too, and that's why the SSD is so expensive.