Vacuum or Mechanical Secondaries?

The difference between a well tuned mechanical and a well tune vacuum secondary carb is only going to be a .1 or two. The vacuum secondary is going to be easier to set up for a novice, is more tolerant to being oversize and will give you better mileage.

For a street driven car there is really no reason to use mechanical secondary carb. Get a Holley 670 Street Avenger or an Edelbrock 650 or sort out the carb you have.