Plot: B5, by far. It fell into a hole in the fifth season, of course, but for me DS9 fell into a hole much earlier and didn't recover. YMMV, of course.
Cast: Hmmm. B5 had Jurasik and the superlative-beyond-belief Katsulas, who were (IMO, anyway) by far the best actors in either series. Furlan is a far better actress than she's generally given credit for. DS9 had Robinson, Auberjonois, Meaney and Visitor. Both series had some truly dreadful actors as well. This is the one area in which I reckon DS9 almost shades B5.
Commander: B5, by far. I detested
Sisko. I could envision wanting to follow Sinclair and Sheridan anywhere. I wouldn't follow Sisko into a room, never mind anywhere else.
Villains: B5, by far. The Shadows had a credible motive. The Dominion's was pathetic - it basically amounted to little more that "they're mean to us because we're different, so let's kill 'em". I didn't find that anywhere near as interesting or compelling as the Shadows.
The thing is, at its peak, B5 captivated my attention & interest more than DS9.
Bingo. For various reasons, DS9 just didn't do anything like as much for me as B5 did. It never put me on the edge of my seat, never made me feel I had
to know what happened next. I did enjoy the first few seasons but after that...eh. To each their own but B5 compelled me to keep watching. DS9 did not. Simple as that. Or, simple as this:
DS9 obviously benefits from superior production values, but B5 at its peak was far more compelling storytelling.