750 AVS or Thermoquad?
In short, I like the TQ. But read on.........
For tunability neither of those carbs...
Ignorant statement.
Innncorrect to a large degree. And this is why;
Both carbs (Assumed OE units) are very tuneable. Both have 3 step rods for idle/driving down the road, part throttle/acceleration, and W.O.T. (Wide open throttle) But there OE carbs. There only so flexable.
OE Holleys are less tunable but very customizeable. So, while it's possible to make a better carb out of a Holley, it'll cost more since you have to purchase parts over the base purchase of the carb. Then of course, you can allways go out and spend 3 times the amount for a really nice holley with all sorts of trick things built into it.
Between the two carbs, the AVS and T-Q, you can do very well. Newer T-Q's have an edge if your not going to radical on the engine build. They came with Alttiutde (SP!) adjuster/compinsaters (More sp!) to enrichen or lean out the carb. So driving from the shore lines to up a few thousand feet, the carb stays in tune. It may not be the best, most accurate, but they worked preety good for there design and non-computered/metered help.
The cave eat to the TQ/AVS, is the lack of metering rods to tune them up real well. You'll have to search/hunt them down and probably purchase a used carb with the metering rods intact. I had to do this, and still do this for my TQ carb(s)
While you can use/adapt AFB rods, they are shorter and you will need the AFB rod covers to swap onto a AVS for there use. In doing so, you gain and loose tuneabilty. The gain is in selection of rods, the loss is there only 2 step rods.
These rods can also be bent (Shortened) for use in a TQ. You need to shorten them 3/4 of an inch.
While I like both carbs and I have NO issues with Holleys, the driveabilty / mileage/power output from any carb relies on the abilty to tune them well. I have had great sucess with Holleys and Carters. But the OE Carter carbs (again) are only so flexable and custmizeable. Once you start running big cams, there hard to tune in and get working really nice. At this point, you might want to look at a nince, upgraded Holley, far removed from a basic Holley to bolt on and use.