Hate to give you a cop out answer but, in my opinion, dead even.
2.5" 66 which is P&R but needs original grips, or a 3" 13, both very desirable. Different barrel lengths would change the dynamic entirely but if you have the means and the want, get them both, they are winners.
I would take the M13. Fixed sights and 3inch barrel for full length extraction and a bit higher bullet speed, plus I like blue over stainless in most cases.
What TAC said. The 13 is IMHO the perfect carry revolver; .357, fixed sights and 3". I don't really like SS revolvers but they are preferred for carry due to their greater resistance to corrosion. Also, wear shows up less on a SS gun as opposed to bluing. You don't see too many SS guns advertised with "holster wear." That 13 is one of my grail guns.
IF you only getting one, AND you are going to use it as your EDC, SS wins. At least for me, where I live.
Anything else, requires constant care. In fact, I don't carry my blue guns if I know I'll be outside all day during the summer. 'Course, I'm at sweat hog.
Personally I like the M13 because I do not like adjustable sights on a carry gun. The extra 1/2" barrel length also aids (probably about 50 to 70 fps) in speeding up a Magnum load. Actually the ultimate to me is the M65 which is just the stainless version of the M13. While I am no big fan of Stainless Steel, I do like it for CCW purposes.
The M66 RB 2 1/2" is a real hot looking gun, but I always though the M65 had the edge over it for actual CCW with Magnums.