I can only speak about the SP game. STW had great atmosphere, but was mainly about the battles. I found the campaign level was a bit a of a grind, to be honest. Conquering Japan, province by province, turn by turn - it was total war, alright and more challenging than any TW campaign since. But overall I prefer a game where I can turtle, go for "glorious achievements", play diplomacy, crusade, nurture generals with their own traits etc. MTW provided that - it gave you more variety of settings and more strategic options on the campaign map. I think the growth of PBMs with MTW shows the possibility the game provided for a "role-playing" type experience.

I also found the STW campaign suffered from more obvious gameplay flaws - the suicidal leaders and the Hojo horde made several of my campaigns dull after a point. The MTW campaign had its own problems - early AI armies of peasants/ballistas and the "I win" strategy of a global trade network - but these loomed less large for me.