I guess trail riding in western saddles in more common in the US, but here in Australia many riders just do trail riding and even short endurance rides in English saddles. It's just the done thing really. You don't even think about it because it's the only saddle you really ride in. Learning at pony club we'd even do a little basic cattle work in our dressage saddles. As a kid I'd ride out for hours, sometimes riding in a jump saddle, but usually a dressage one! You'd probably find that while learning, whether english or western, you'll end up doing circlework and arena work for a while. That's where you'll learn your control, where the instructor can watch you and teach you, and later you'll have the skills to ride out. After all you can go to many places and go for an hour trail ride if that's what you want, but you're going to a riding school where they actually teach you to ride.
Two point, well you might not find yourself using it a lot but it's teaching you balance and you use your balance all the time in any discipline.
I think its good to learn English first. I learned english and when I went into a western saddle it felt a tad different but I would still ride the same, and never touch the horn, all in all a very easy change Where as the other way around, peoples seem to find it a lot harder to switch to english.