My two cents: All of the products mentioned above are obviously abrasive, hence their ability to "polish." I think what works best for one has a lot to do with what one is attempting to accomplish. If you are attempting to polish out some minor dings in the brass, you probably want to begin with one that is more abrasive and that has always been an auto rubbing (not polishing) compound for me. If the brass is really in bad shape, you may even need some fine #600 wet/dry sandpaper first. You can then work your way through progressively less abrasive products. I think that Flitz, Simichrone, and MetalGlo all give comparable results, but all tend to leave minor "swirl" marks, at least for me. For the final treatment, and if you want to give a mirror like shine to otherwise perfect brass, I find that Mothers' Mag and Aluminum Polish (available in auto parts stores) is almost impossible to beat. Light rubbing in the same direction with a damp cotton ball will leave a near perfect finish. Now, when you are finished, if you want to prevent further tarnishing, try applying a very light coat of lacquer with a small artist's brush to the bolsters. It dries almost instantaneously, you cannot see it after it's applied, and it will definitely prevent further tarnishing. It will, however, wear off if the knife is a user. Furthermore, the lacquer can easily be polished off with almost minimum effort. Just be sure to mask off the wooden handle scales. Most brass products (lamps, bookends, etc.) for use in the home are finished in this manner to prevent them from tarnishing as well.
Edit to add: All of these products will remove blade coatings with vigorous polishing.