Steven posted a small teatrise that lauds Strike Witches and pits it against Sekirei:

But the real reason the [Strike Witches] works for me is the characterization. All the girls are good, but the characters of Miyafuji and Sakamoto make the show. They're both really interesting, and the interaction between them is also really interesting.

[...] And that's why it's outselling Sekirei: that one doesn't have any equivalently riveting characters.

I think it cannot be right. It's a grown-up's perspective, and very off-center for the buying demographic in Japan. We may posit equally well that it's lesbians who sell Strike Witches: the way its cast falls into neat pairs is a shipper's heaven, missing from Sekirei. In any case, we'll never know unless we run some focus groups.

But I just wanted to muse, why is it that I enjoyed Sekirei more, and stuck with it to the end?

On reflection, it comes down to two things: Tsukiumi and fanservice.

Fanservice I already touched upon after 3 episodes:

[C]oming from Strike Witches, Sekirei offers extremely soft and polite fanservice by comparison. In Strike Witches, the key idea is to pound into viewers’ minds that no matter what is shown on the screen, it’s completely natural and acceptabe in the other world... so, tough it out, suckers! But Sekirei returns to standards of fanservice seen, for instance, in Ranma (actually, it is milder in the early Sekirei, but I heard it turns the nipple knob up later quite a bit).

Sadly, Sekirei degenerated in the second half; the DVD-selling steam started to pop up regularly, etc. Still, it remained way and way ahead of Strike Witches.

As far as characters go, Steven is right when the issue is considered rationally: there's nobody in Sekirei with a well developed, "riveting" tragedy (in the first half at least). But Squirtle was too charming (and of course she's ultimately doomed, absent some trick by the author, like a group ascension for the winning Ashikabi). I am pretty sure I crossed the bad parts only to see how she's doing and if she's going to last until the end of the season.