Definitely both.
While SII has a better story, SI has more epic war battles, a better HQ, and among other things. SII takes place years after SI, so there are many returning characters. The two Suikodens have a very similar style, so if you like one, you will like the other, might as well start with SI.
--
KOS-MOS: There is no connection between my current systems and your life functions.