I'd like to add 16 G of ram to my xps 8940 for a total of 32G

I bought it with

dell.com who has detected my pc suggests

Dell Memory Upgrade - 16 GB - 1RX8 DDR4 UDIMM 3200 MT/s

     or         

Dell Memory Upgrade - 16 GB - 2Rx8 DDR4 UDIMM 3200 MT/s

which would be best for my pc and what slot should it be placed in.  (currently the 16 G is in slot 2)

thanks