Get rid of your engine guy if he told you a supercharger is full boost all the time and a turbo is only making boost when it's spooled up. A supercharger makes more boost as the rpm's go up also, and can be underdriven to whatever your engine can handle.
Neither will work if the static compression of your engine is too high to start with. And forget fuel economy completely. If you're trying for 500 HP you're kidding yourself to even think economy.