md RAID, the only constraint I'm aware of is that the new drive must be no smaller than the old one. For that reason, I often move to the next drive size up (ie, last time a 1TB HDD failed, I replaced it with a 2TB; sacrificing the extra blocks was less time - and therefore money - than agonising about whether the new 1TB drive was 20,000 blocks smaller than the old one).
In your case, that would mean moving to a 3TB HDD, and I understand there are some fears that these are not yet completely reliable.
So: either get a definitively bigger drive, or get another 2TB, but make sure it's no smaller than the old one.
smartctl will tell you exactly how many bytes your current remaining drive can store, and the vendor's web page should tell you about any prospective replacement.
In the case of the two drives you mention, the vendor site says that both have the same number of sectors "available to the user" - 3,907,029,168 - so I think you should be OK with your alternative drive.
Edit: for hardware RAID it will depend on the specific requirements and peculiarities of the hardware in question, and that makes the question much more difficult to answer. Your hardware vendor is your best port of call in such cases.