It appears to be a choice between the 128GB SSD and 8GB more system memory.
I would definitely go with the 16GB model.
I would assume you can add an SSD later (M.2 2280 PCIe ??) since they likely share the same exact motherboard etc.
*I do recommend you PARTITION the 2TB HDD so the Windows (C-drive) partition is 200GB. Ask for help or Google how.
That way you can for example make a STEAM partition on the 2nd partition (E-drive) which makes it very easy to add a 250GB SSD later and CLONE the Windows partition over.
So...
1) partition the 2TB HDD (200GB, possibly 450GB if adding 500GB SSD later to put some games on the SSD)... 2nd partition is rest of space
2) buy SSD later
3) CLONE Windows partition over
4) boot to BIOS (sometimes Power + F2) and change boot order to SSD
5) TEST for at least a week
6) use Disk Management to delete, then recover the HDD Windows partition
7) make a compressed backup IMAGE of the SSD so you can RESTORE it in case of SSD hardware or software issues (Acronis True Image or similar)