As someone who has extensive experience with both the P6 and P226 I will add a few comments.
As already stated the P6 is the German police version of the P225 which is a compact version of the P220 in 9mm. There are some differences between the P6 and P225. First and probably foremost is the trigger. The P6 has a heavier pull. This can relatively easily be remedied by replacing the P6 mainspring with a standard P225 mainspring available From Wolff Springs. If you can push a pin you can do it yourself.
Next, the P6 feed ramp profile in earlier P6 is such that many have reported having difficulty feeding hollow points. My P6 is a 1978 and has the earlier profile and has never had difficulty feeding hollow points (I use PowrBall nonetheless). SIG changed the feed ramp profile in June 1989 to accommodate hollow points. Also, the P6 does not come with three dot contrast sights having plain black sights that that to me are not optimal for quick sight acquisition. I remedied this by installing a Trijicon front sight. Finally, the hammer is different on a P6 having a notch cut away that many surmise allows an armorer to know if the gun has been dropped on its hammer. I'm not sure if the P225 has the more recent rebounding hammer, but the P6 does not.
One thing that I really like about the P6 is that it is the earlier stamped and pinned slide. Many SIG folk such as myself prefer these earlier guns. Myself, I find it to be a matter of balance. It is quite possible that the P226 that the OP is looking at is also stamped and pinned which would be a big plus to me. Look for a roll pin through the serrations on the slide and an internal extractor for a quick ID of the pinned and stamped.
I consider the P6 and P226 both to be outstanding guns, about as good as it gets. In choosing between the two, condition being equal, it would depend what you are looking for. If you want a higher capacity full size duty pistol the 226 is the way to go. If you are happy with eight plus one rounds and are looking for a pistol that potentially could be carried, then the P6 is the way to go. To me the choice is a coin toss, one in which I might be hoping that the coin falls in favor of the P6.
As to buying a P6 or used 226, I would want to inspect the gun in person. While my 1978 NW had some slide wear and was like new internally, I was able to determine that in person. To me an actual inspection prior to acquisition is the A Plan with someone choosing a gun for you somewhat of a crapshoot despite the P6 generally being very nice overall.
I have recently acquired lightly used P6 mags in the $30 to $35 range.