The series build will be pulling around 10 amps from each battery, and the parallel a touch over 20 amps from each. The 25R is a 20 amp CDR cell so you are good with both but better with the first as it will stress the
batteries less.
The NC 2-25 is my daily banger, I exclusively use it in variable voltage mode as this gives more consistency and flexibility for me. I use Samsung 30Qs in mine as they also have a 20 amp CDR, but a longer run-time with 3000 mAh rather than the 2500 mAh of the 25R.
I used used
steam-engine to calculate this. Along with
Mooch's recommended battery chart you could do this for yourself next time.