If you have something like unRaid then the redundancy is built into the system. So if a single drive fails you shouldn't lose anything. With that you can have 3tb or whatever and it'll mostly be ok. You do end up with a parity drive that can't be used for storage though.
It's a matter of weighing up the pros and cons of it all I suppose.
Yeah, it all boils down to your potential expansion plans and capacity requirements - if you've never filled a 1TB drive it's pointless getting a 3TB i'd imagine, might as well get 2 1TBs and have a backup. The downside with unRAID is having to upgrade the parity drive first before you can upgrade the others, but it makes sense I suppose long term.