Yeah, genuine leather are very expensive. Actually synthetic leather is also durable already, unless your car worths hundreds of thousands, don't bother install very expensive parts.

Actually Napa is not a brand, and it is overly abused term (like Satay Kajang, they are everywhere).

Yes PU is synthetic/faux/composite leather.

They are both susceptible under the sun, but genuine is more durable. If your car got good tint and reflector, and windows rolled down a bit when parked under scorching heat, I think shouldn't be much of a problem.

Napa is a 'type' of genuine leather (provided that it is not fake). Given that, it is the most famous type of leather in automotive.

And if you go for genuine leather (including napa), it is quite costly to maintain them, although not that often. Whereas PU is much easier, almost any aftermarket detailing products (even those being sold at Mr.DIY) can do the cleaning and restoration well.

Not napa, napa leather has this grainy yet smooth texture. Those real smooth textures are synthetic, as there's no animal hide that is truly smooth, so SS one is likely to be one.