If you do POP right, then youll get a good finish on your baits.
I like the aluminum injectors - easy to use and almost perfect baits every time - just dont use too many pounds of pressure and youll eliminate flash on your baits. The thing i dont like about aluminum, is that if your only working with one mold, it takes more and more time each time you inject for the bait to solidify. If you get it out too soon, the inside of the plastic is still hot and gooey and the bait doesnt form right.
2 piece injectors are nice, but if a small drop of plastic of gravel gets imbetween the mold, you can bet that the next injection is going to be fail because of excessive flash.