As Szkom has said, the rear "brace" only stiffens the rear bulkhead between the two rear seatbelt mounts (which was hardly a flexible section of the bodyshell anyway). It is essentially just a bit of cosmetic bling and serves no useful purpose (unless the purpose is to add cosmetic bling....
)
The upper front brace is very worthwhile - it certainly adds quite a bit of precision - you will feel it most in the form of sharper turn-in, especially when changing from one direction to another (as opposed to just turning in from a straight line). As for brand - either the Sparco brac fitted to LEs and Pluses, or the aftermarket OMP brace