I strongly recomend 3TB. Remember that HDD on the iMac are not upgradable. The case is near to impossible to open and the HDD itself has a identifier chip, so its not replacable. If you swap, the iMac doesnst boot.
On a $3000 machine, the diference for HDD is minimal, and you really gonna regreat not having more storage once that 1TB gets full.
Memory on the other hand is perfectly easy to upgrade, you can do it yourself without any issues
Or maybe invest in a NAS or other external expandable storage solution instead of spending more on a hard drive. PS, the hard drive in the 27 iMac appears to be very easy to replace if someone chooses to do so.