All carbs have pros and cons. Just some warnings, and some advice.
Quadrajets are a one-size-fits-all. You'll need to jet it, set the idle screws, idle, and depending on your car, you may need to adjust the secondaries to prevent bogging.
Taking a Q-jet apart takes some knowledge. You'll need to read a little on how to disassemble one, and how to reassemble it properly, or you'll have all sorts of tuning problems. Even a new rebuilt one may need to be taken apart to fix various issues. Just like any carb, you may need new gaskets when you disassemble it. Never hurts to have a rebuild kit just in case.
Holleys are a little easier. Parts are plentiful, gaskets can be found at any auto parts store.
My .02 cents ; since you already have the Holley 650, and it just needs to be rebuilt, this would be a great opportunity to take it apart and rebuild it, and get to know a little about what makes them tick. It will also be much cheaper than buying a rebuilt Q-jet.
