the supercharger is a bolt on deal. for about $1800 to about $3000 you can get a bolt on supercharger that will give you anywhere from 35% to 55% of power. very simple installation, you are done in one day.
twin turbos will be about $3000 and up. the install is very complicated because everything is custom made. it will take you about 3 weeks( more or less) do complete and have running.
8 psi of boost in a supercharger is completely different than 8 psi of boost on twin turbos.
yesterday my friend raul iglesias with a stock 302 FFR with twin turbos, 30 lb injectors, 3:08 gears, a very simple set up, defeated a diablo by 1 1/2 car lengths in 3rd, 4th, and 5th(he missed 5th and still was ahead). he o-ringed the heads and boosted 10 psi.
either way you go, it will haul ass!!!!!