I recommend fully weaned birds every time.
The idea that hand-feeding the bird yourself will give you a stronger bond with the new baby is misleading and dangerous. It has cost the lives of a lot of baby parrots. Hand-feeding is a skill that improves with practice and experience. It is hard to learn from a book. And doing it wrong can cause a variety of problems, some of which are potentially fatal. It is not a simple task for a beginner.
More importantly, it is not necessary for bonding. Early handling and positive experiences with humans are important to encourage a baby parrot to trust humans. If handled correctly at a young age, babies will learn to tolerate gentle handling and hopefully not fear hands. This is a big part of why hand-fed babies tend to be more sociable and easier to tame than parent-raised birds. But a good breeder should already be providing those positive experiences for your bird. And you can build off those good memories, whether you are getting a baby, fully-weaned bird, young adult, or even a fully grown bird. You do NOT need to be part of the hand-feeding process to solidify the bond.
I do think it is very important to learn as much as you can about hand-feeding and gather the necessary supplies before you get a baby parrot, even if it is coming to you fully weaned. Sometimes baby parrots will revert under stress and require additional hand-feedings. And sometimes they are "force-weaned" and would benefit from being allowed to hand-feed at liberty until they are truly ready to wean completely. And also, teaching your parrot to accept liquids from a syringe (and learning how to feed them from a syringe safely) is a very useful skill, should you need to administer liquid medications at some point, so it is nice to have the supplies available, even if you don't end up needing them.
But as far as intentionally getting an unweaned baby to bond better ... nope. Don't do it. It is not necessary and could go horribly wrong, especially if you are going in "blind" with no prior experience with hand-feeding techniques.
Breeders that encourage buying unweaned babies or only sell unweaned babies are, at best, passing on bad information. At worst, they are intentionally encouraging a dangerous practice, so they can sell babies faster and at lower cost to themselves.