The smaller drive because it has the higher costs.
The magic of unRAID is low cost. You acquire (discard) storage space based on cost. When additional storage is needed, shop for $/TB and install. The operating costs are very similar for most drives, with green drives being X regardless of size and 7200 drives being Y. So replace the smaller drive will get the best return for the new drive purchase.
I know people try to match all the drives. But really a lot of effort has been put into unRAID so that you can mix drive sizes and expand over time.
If you want (24) 2T drives with RAID6, hot spare, etc, check out mdadm, a core Linux function. What's missing? an ability to use mixed sized drives and easy expansion.