I've been riding 390s since the first year they came out. When the Boss came out last year, Rome upped the ante to an even higher level. If you like loads of adjust ability, comfort, and bindings that don't quit, these are them. If parts ever break, Rome is quick to hook you up, as well.
In terms of size, I would go S/M in a size 9.5 and smaller boot. The only brand that is an exception to this rule would be 32, where I would go s/m for 9 and down. If you are rocking a slim profile boot, such as the Salomon F series, or DC parks, I've had 10s in the S/M. Hope that wasn't confusing. I like to point this out because although it says you can put a 9 in L/XL, I would say it is far from ideal.