I'm pretty sure they all use the same Oracal vinyl (at least Cubesmith and Puzzleaddictions do), so it's all basically the same sticker quality. It all pretty much comes down to the "perks" of the store (shipping speed, range of products, etc.)
Personally, I've only ordered from Cubesmith and Puzzleaddictions, and I was satisfied both times. And from what I've heard, SubXX is a pretty great store, too. Pick the store that best suits your needs.
SubXX. They have Bright red that i like, that cubesmith doesnt have. SubXX company, thecubespecialists' shipping is much more quicker than Cubesmith considering it is all the way in Germany.
Never heard of subXX, cubsmiths postage isnt the best, but generally I/you should/will be ordering enough stickers to last you a year or so, so you can just reorder a few months in advance.
Sub XX have really cool stickers and I think the 3x3 sets are designed to fit dayan cubes but cubesmith may have better quality (Just ordered a whole bunch of cubesmith stickers)