George,
See "Performance Enhancements", Snowman's post, I just ended up discussing the mechanical vs vacuum issue though these guys covered it well.
I've seen Panic's situation many times, where my best running carb on the clock was really too big, but the engine just "liked" the way it was tuned. But a carb that is the right size AND tuned right will be best, whether mechanical or vacuum.
Vacuum carbs are a lot more forgiving of too big of a carb, because they will just "decide for you" to only open the secondary part way since that's all the air the engine wants. Their disadvantage is the same - without careful tuning, they can open the secondary too early (an air bog) or too late (starving the engine for air/fuel). Mechanical carb is more "certain" but you can't just floor it and go - YOU have to put your foot where the engine wants it which is not as convenient as a vacuum carb.
I like and actually prefer mechanical secondaries on a stick car, but I like a well-tuned vacuum carb on an automatic - once it's set up properly, you're done.
But, if I find either type for under $40 and it's a good runner, it "becomes" the right carb, for the time being at least.
