SPR build

I'm getting ready to either purchase or build a SPR upper and want to hear your thoughts/suggestions. I'm looking for light weight and accuracy.

If I buy a complete upper I am either going with a LaRue or a BCM....I like both although I lean more towards the DD Lite rail over the LaRue.

If I build this is what it would most likely be....

18" WOA fluted barrel (Mid)
Rainier Arms Billet upper
12" DD Lite Rail (Mid)
low pro gas block (haven't decided)
BCM BCG
BCM Charging handle

Also, what are the main differences between mid length and rifle length gas systems?

Lets hear your thoughts please.....