Screen size, S9 is larger and comes in handy if you wanna watch wide screen videos and also read txt files.
D2+ can max out @ 48GB with a 32GB SDHC card. S9 is stuck @ 32GB max since there is no SD slot.
S9 uses a USB cable that is only standard in Korea for charging / connected to PC. D2+ uses a typical USB cable. For the S9 you need to specifically buy a Korean standard USB cable if you want a spare or loose the included USB cable.
I chose the S9 over the D2; D2+ was not out at the time.