Both the 686 and the 625 are excellent. I've owned both and been very pleased; I sold my 686 in a moment of weakness and the new owner frequently reminds me of what a great gun he got.
As far as accuracy goes, either gun should be able to put a full cylinder into 2" at 25 yards with the right load. With most defensive ammo, 4" groups at 25 yards should not be a problem.
But I'd be more concerned with practical accuracy than with mechanical accuracy. If you don't have big hands, you'll shoot better with the 686 because the shorter trigger reach allows better leverage. I have average-sized hands and I can clear a table of bowling pins with my 625 in reasonable time, but if I have to shoot one-handed, the 625 doesn't have the comfy "extension of the arm" feeling that the 686 does.
Another advantage of the 686 is that practice ammo is cheaper; .38 Special ammo sells for about 15% less than comparable .45 ACP ammo. With the 686+, you can get seven shots in the cylinder versus six for the 625.
The 625 does have the advantages of being a little more intimidating and a good deal quicker to load and unload with moon clips. True, you can't do a "tactical reload", but in a home defense scenario you shouldn't need to conserve ammo.