Well I live in Canada, so the whole Limited vs Premium thing is a little different. (Ours are just the base BRZ, and BRZ Sport Tech). With the Premium (or Sport Tech), you get heated leather/alacantara seats, push button start, fog lights, spoiler. Everything else is the same. HIDs are common to both the trim levels of the BRZ.
As far as waiting till October goes, if you want to get in and enjoy breaking the car in during summer and get used to it before the winter comes, get it now. If you have another car that you can use and don't mind waiting till October or later to get one, and don't mind starting driving it in winter basically, then you should probably wait.
I'm in the same boat as you right now, I could get a 2013 FR-S now, or wait for the 2014 when it comes out in fall. I don't know how exactly Subaru works, but with Scion, you could order the 2014 parts you like and just have them installed into your 2013 model by the dealership, so I may just go that route. If Subaru does a similar thing, then get a 2013 and start driving it now.
