For me, the 3” barrel is pretty much perfect. Just long enough to have a full length ejector rod. I don’t think any of the 3” variants were early enough to be pinned and/or recessed. My 4” Model 66 no dash has both features.
If you want to look at it, the earlier 2-1/2” version may cost less and look great.
Unfortunately the prices the 3" -2 and -3 are commanding almost make them unshootable, almost lol. The longer length ejector rod is certainly a nice feature as well as excellent balance. I was too stupid to hold on to the one I bought when they first came out.
I don't think you give up much in the way of concealability nor comfort when carrying with a 3 inch barrel K-frame, but you gain a full length ejector rod. The 3 inch K-frame is probably the best for defensive carry, sadly, S&W made far more with shorter barrels. I'd pick the 3 inch, but they tend to carry very high price tags these days.
To me, a round butt K frame is all about concealability, so the 2-1/2" is what I want. I don't think 1/2" more barrel would make the gun more accurate at the distances you'd use if for anyway. I like the P&R features, too.
I had a 2.5 for many years but sold it when I found a 3" NIB several years ago. I like the longer sight radius and the full length ejector rod. I also have a 3" CS-1 686 and a 3" 696ND.
I have owned (and continue) to own many 66's. At one point I had 7. Now its down to 4. I love the utility of the 3" model 66's but love the look of the 2 1/2 inch guns. I prefer stainless revolvers but lately have branched out to a 36, a 15 2" and a 4" 19-3.
I'm picking up a 66-1 2.5 inch snubbie tomorrow. It is filthy but looks like it will clean up pretty decent. I think my Guy Hogue stocks will look more appropriate on it than on my 686. Box included which is a rarity around here.
Given the choices the OP provided. I would take the 3" 66 if they were both in similarly great condition and priced reasonably close. Basis for that decision would be the scarcity of the 3" guns as compared to the 2 1/2" guns. Also, as you can see, those that have 3" models tend to keep them. The last two model 66's that I purchased happened to be -4's. One 2 1/2" and one 3". I must admit though. I don't know how many P&R 66 or 66-1's were produced with 2 1/2" barrels as compared to the 3" gun quantities. I also do have a 66-1 2 1/2" so I may be a bit less concerned as collections go.
I'm picking up a 66-1 2.5 inch snubbie tomorrow. It is filthy but looks like it will clean up pretty decent. I think my Guy Hogue stocks will look more appropriate on it than on my 686. Box included which is a rarity around here.
As with so many readers here, I am pleased to have the 2 1/2 inch, the 3 inch, the 4 inch and the 6 inch.
For me, the 3 inch is so close to the 2 1/2 in a small holster that the difference is negligible; but the handling at least for me is far better with the 3 inch. Similar to those seen at post #9 above, mine wears some of Big mountainman’s fine elk stag grips - a little thicker than normal to allow for a firm hold.