For a mild small block street engine, there really is no clearly superior carb. Both have their particular issues and both can be made to work well and in the end, if properly tuned, will perform equally. The biggest difference when starting to tune the two is flexibility and parts availability. The Carters are not real easy to tune particularly in the low speed transitions. And parts can be a problem to find without ordering and waiting. The Holley platform is easier to access the points that usually need to be addressed when tuning and parts are everywhere. Heck even my local Oreilly's has Holley parts on display.
It is really important to start with a carb that has as close a tune as your engine needs to start with. The farther the original tune is away, the more fiddling and more tools and knowledge will be required to get it right. But choosing which one that is, is a real challenge. And remember, 40 year old carbs were designed to deliver a very different fuel than they will see today, so even a carb that was designed and tuned for a particular engine in the 60s or 70s, will need some adjustments to run properly today.
If someone wants to tinker with carbs, whether it is a Carter or a Holley, there are certain bare essentials required. First are books. Because a good understanding of the circuits and location of metering orifices is an absolute must. And a good selection of tiny drill bits because may orifices in both brands are drilled, not screwed in. Or you can get a hand full of bleeds and a drill bit and tap to install screw in bleeds when space allows. And at least a narrow band O2. A wide band is much, much better, but a narrow can suffice for beginning tuning. The only advantage to the narrow band is that it is much cheaper. And don't forget time. It will take a lot of time to read and understand the workings of your chosen platform. And then more time tuning and driving and tuning and driving. But all of that is part of the attraction for guys that mess with carbs. While all of that is why most guys don't mess with carbs.
Here is an alternative to forego all of that if learning to tune their carb is not on someone's life list. The Street Demon. This is the best carb I have ever installed on mild street engines.
https://www.youtube.com/watch?v=DPOuEOkazVchttps://www.hotrod.com/articles/ccrp-1209-01-street-demon-carburetor-the-new-625-cfm/https://www.ebay.com/itm/Factory-Refurbished-Street-Demon-1901-625CFM-4bbl-Carb-/351711332709https://www.youtube.com/watch?v=YNAlVFtUFL4
