Thanks for the feedback, m R.

I can see how the track is vulnerable to dirt. OTOH, the windows I
have now are gliders, and I've only seen a need to clean the tracks
once in 3 years, so I don't think that's a deal-breaker.

We chose gliders because they suit the architecture of the house and
there didn't seem to be a compelling reason not to (except for the
difficulty getting a look at them, as documented here.) However, I
will double check the ratings, on your advice. Like the Andersens, the
Marvins seem to close nice and snug when the latches are flipped, so I
don't see a lot of potential for leaking, right off.