I'd say Turbo charging is going to give you more bang for buck, as far as the cost of doing the conversion goes....
as for fuel economy.... I think turbo is also going to be better, at least theoretically you can keep the engine from going into boost if you really need to limp 15miles to the next gas station....with the fuel gauge needle 4 thicknesses below the empty line
been there.... didn't make it though ...