I'm no expert but my thoughts go like this -
But thoughts about your specfic questions are:
While Cuben is light it packs about 30% - 50% bigger than Sil' so I'd firstly decide whether weight or packsize is the real concern, for me it's packsize. Secondly, as Chew says, Cuben is pretty much see through so I'd question whether that's a good thing on a public campsite

You're right about Sil' stretching when it gets wet but it's the ability stretch / give that lets you get a tight pitch in the first place, if your shelter has adjusters on the edge then it should be possible to tighten the thing from inside. I tend to think of the stretch as a benefit rather than a drawback
suffers badly from condensation (is that right, or is it when you touch it when wet water comes through? Either way you can get wet). Never had either of these problems with the Lair but your right it could be tricky to pitch.
Red Herring I'm afraid ... neither are breathable but your Lair had a peak vent and your Trailstar doesn't. The vent will be the reason why the Lair doesn't suffer rather than the material.
The Solomid looks really good (I like 'mids) and the quality will be top notch but if I were spending my own money I'd go Sil' ... on paper it just makes more sense to me.
Sil': 440g £110 + packs smaller
Cuben: 298g £200
I'd also be taking a look at the SMD Gatewood Cape 312g £118 and the SMD Wild Oasis (bug proof) 369g £153 ... both available in the EU so no charges to pay.
However ... there's no getting away from the Cuben geek factor
