Over the years I have kept coming back to the 5 inch 27. Initially, I was attracted to it by the writings of Skeeter Skelton in Shooting Times magazine. I turned 21 back in the mid seventies and being legal for handgun purchases I just had to have one. I'd say Skeeter is to the 5" model 27 as Jack O' Conner was to the 270 Winchester.
I have owned all barrel lengths with the exception of the 8 3/8'ths. Presently I have 3 1/2, 5 and 6 inch. My favorite, the 5 inch. It balances much better in the hand. I shoot it better. That's the bottom line for me. The 3 1/2 " is possibly the best looking, most business-like of all the S&W models, but to me it feels heavier in the hand due to lack of barrel weight out front. I would chose the 6" over the shorter one also. The 4 inch model is slightly less "weighty" in the hand, but it cannot compare with the 5" for balance.
I have always managed to find the 5 inch version when looking for one so I wouldn't say they are rare. Lately I don't see as many but they do come up often enough here in our classified section.