Which Carb Gives a Balance of Power & MPG?

I'd stick with the carb you've got. You would have to drive a lot of miles and get a big difference in gas mileage to justify the expense. Probably the same is true for a wideband, but I still recommend getting one.

I like the Holley/Demon stuff. I would choose a street avenger over a demon, but that's just personal preference. Power is typically better with Holleys than Edelbrocks (though on a mostly stock application there's probably no difference at all).

Something very important to remember is that timing (vacuum advance, etc) plays a significant part in fuel mileage and power. Getting that dialed is important.