Some saddle questions?

I'm planning on investing in my own english saddle soon. I'm undecided between a dressage saddle or a close contact jumping saddle. I do NOT want an all purpose (all purpose = no purpose, only English saddle I have to ride him in right now), and I can't buy both. However I will be practicing both disciplines in the saddle, along with cross country, possibly hobby endurance (could take my western for that).

My current horse is a shagya arab, he has a thin barrel and prominent withers. I'm planning on riding him to several tack shops to try the saddles (will arrange with the shop first of course). I'll have an experienced person with me, but I'd like any tips from you guys in terms of how to tell that the saddle fits well in a quick test fitting/ride?

Last but not least, the girth strap. Made out of so many materials, which do you/your horses prefer?

Thanks. :)