3-1/2" and 5" barreled revolvers are usually older than most 4" and 6" guns, and if the conditions are equal, they go for more. Used to be you could order any length in 1/2" increments, but up to 1979, the 3-1/2" and 5" were the standard catalogued lengths, and after that, 4" and 6" were the catalogued standards.
In the case of the M29, the 6-1/2" barrel (D/C'd in 1979) brings a premium over the 6" barrel, although the 6" barrel is more rare.