Sorry, English is not my first language... What's a polo?
http://en.wikipedia.org/wiki/Marco_Polo_(game)Is there any real difference between different manufacturers? Would that option leave me with AF intact?
The filter looks very convenient, could also be used for some macro stuff on vacation as it wouldn't add too much bulk.
Only in build quality - tubes have no optics. Personally, I went Canon tubes because my main use for them is with a heavy supertele lens. For typical macro-type uses, the Kenko tubes are a great option, and would leave AF intact (although you'll see more focus hunting with the tube in place, but that applies to Canon tubes, too).
Both tubes and close-up lenses cause a reduction in IQ, the degree of that impact varies with the lens and the quality of the close-up fliter. A good filter (like the Canon 250D/500D) on a good lens produces results that are practically hard to distinguish from a dedicated macro lens (I compared the 100L to the 70-200/2.8L IS II with the 500D).For a 50mm, extension tubes ... wouldn't degrade the image quality at all.
For a 50mm lens, tubes are a better bet, IMO. It's not strictly true to say there's no optical degradation. By forcing the lens to focus outside of it's intended range, you are compromising the optics a bit, and the additional magnification accentuates the native flaws of the lens, too.
I suppose that's true and it wold have been better to say no noticeable reduction.