I have 2 drives inside a ReadyNAS Ultra 2 Plus running 4.2.15.
My requirement is that if 1 drive fails the data would still be in tact and I would just replace the failed drive to become redundant again. Expansion is not really important to me I am after redundancy.
Both can do what you want. I'd suggest sticking with X-RAID2.
As of 4.2.16 both of these options will be expandable not just X-RAID2. Assuming you have e.g. 2x2TB disks in the NAS you will need to replace both disks with higher capacity disks (e.g. 2x3TB disks, 4.2.16 is required to support 3TB disks) to get expansion as expansion can only take place when redundant space can be added.
While expansion may not be important to you now, I expect it will be in a few years time.
The volume can only expand when redundant storage can be added. So that if any single disk fails data remains intact the capacity of the largest disk is used for redundancy.
In a fully redundant X-RAID setup, the amount of redundancy required is always equal to the largest single disk installed - in the example you show, that means 1.5TB once the 2 x 1.5TB HDDs are installed
