[wkooiman]

Speaking from experience running both on the street.

I had a 67 GT500 replica with a 428 and 2x4s.
I still have a 72 Pantera with a single four and for a while 4 Webers (going back on soon).
I still have a Contemporary Classics Cobra with a 428 and 2x4s.
And, of course, I have had several 1x4 Holley equipped cars over the years.

In general, a 1x4 is better at cornering, and it is cheaper. A 2x4 is visually more impressive, but from a performance standpoint, it's not going to make a lot more HP than a properly setup 1x4.

Webers changed my Pantera. It idled more smoothly, and felt like it added 100HP to the bottom end. Of course, it didn't add 100HP, but it felt that way. Gas mileage dropped 4 points.

2x4s didn't change the car very much. It drove about the same, although you could feel the progressive linkage as the second carburetor started to open. I enjoyed the mechanical connection to the engine.

A Ford FE is impressive with 2x4s and S&H air filters. When you look at an FE with 2x4s, you say, "Wow!" When you see a 1x4 with a turkey pan, you say, "Nice!" When you look at a 351W with 2x4s, you say, "What is that?" At least that's my impression. An FE with 2x4s is instantly recognizable. A 351W with 2x4s, not so much.

2x4s are not too much carburetor. They are typically vacuum secondary, so it's like a 600 double pumper, with another 600cfm if you need it. If you don't need it, they won't open. Too much carburetor with 2x4s is a common Internet misconception.

2x4s got good gas mileage on my 428 Mustang. I think I was around 18 or 19mpg (that was a long time ago). When you are cruising, you are only on 1 carburetor, so there's no real difference between 1x4 and 2x4.

At any rate, that was my experience.