tarptent rainbow is huge inside, because it has the arch going the long way, almost all the space inside is usable. It's also very easy to pitch, and takes little room on the ground. However, cross winds are probably going to blow cold air through the tent because it is still basically a tarp-tent.
It actually feels sort of ridiculously huge to me.
I don't know if tarptents are easy to get in europe though.
I believe, based on that data provided by European tent companies, that the silnylon they are using is significantly more water resistant than the stuff being used in the USA, even the shield stuff. ie, 5000 mm hydrostatic head, and in heavy rain, that's a big deal, so you might want to verify those numbers, the US stuff seems to be around 1500 or so, not sure what the shield sil that Henry of tarptent sources is officially. Maybe 2000 I'm guessing?