I have a 66-2, 2-1/2" and love it. There is something comfortable about a K-frame. To answer your question, maybe because they're worth more and perhaps rightfully so, I'd lean toward the 3", though.

The reason I prefer the shorter barrel M66s is concealability. As you know, however, the longer the barrel, the greater the energy and velocity, so the longer barrels take full advantage of .357 Mag over .38 Spl. The difference between the two calibers in a short barrel is just not that much, so I rarely shoot magnums in my M66. And there are reports that forcing cones have split in M66s, especially shooting lighter bullet .357 Mag SD loads.

Where this is going is that if I wanted to shoot much .357 Mag, especially hot loads, I'd want a 4" or 6" barrel to get significantly improved performance over .38 Spl. And for that I'd lean toward an L-frame M686 for its extra frame strength and full diameter forcing cone. Also, the heavier M686 is more pleasant to shoot the spicier loads. But if I wanted a very accurate and comfortable .38 Spl +P revolver, a M66 6" would be hard to beat.