The only difference here is the Battery capacity, SOC and RAM
UMIDIGI S2 offers a 2.3 GHz MediaTek Helio P20 chipset and 4 GB of RAM. While the older model with the PRO prefix can boast a Helio P25 chip with a frequency of 2.6 GHz in conjunction with 6 GB ultrafast LPDDR4X memory.
Speaking of the Battery S2 is powered by 3000mAh Li-Ion (Lithium Ion) battery Type. While the older model with the PRO prefix S2 pro is Powered by 5100mAh Li-Ion (Lithium Ion) battery Type.
Read more: Official UMiDIGI S2 and S2 Pro Full Specifications and Prices
6 gig ram makes almost no differance, it is all about that most people are thinking more ram is better why is a I Phone with 2 gig ram faster teh android phones with 6 gif ram?
it is all about software optimisation, not the hardware !!
i can tell you that a phone with 128 gig rom and 4 gig ram will be faster, so i think a bad choice