I've only seen about an episode of each! But my reasoning goes like this: in Merlin fic, people use actual (non-slash-related) plot points in episodes; they write about one-episode guest stars; they write what are essentially "casefile" fics. As far as I can tell, nobody ever does that in Sentinel - they just seem to write relationship-focused stuff, to the point that I couldn't tell you a single plot point from any Sentinel episode that doesn't bear directly on Jim & Blair's big gay love. Whereas I could probably give you a pretty good summary of all of Merlin so far. (Though this may just be because Merlin doesn't *have* plot points that aren't about big gay love.) So Merlin's writing must have something going for it that Sentinel's doesn't.

(Also, I think I'm more forgiving than a lot of people of the way Merlin-BBC relates to the mythos. Compared to some of the stuff I read the last time I was in King Arthur fandom, the Slash Dragon is a nuanced and accurate interpretation of the original story.)