Unfluted Cylinder
I have a 4" and a 2 1/2", they are awesome. When revolvers had names there is a reason that the 686 is the Distinguished Combat Magnum. There was a 3" series made for US Customs a while back before they went to autos (CS-1 model), but I could never locate one.
The 3" would be perfect for me, but an unfluted cylinder is a no go for me on a defensive gun.
On a defensive gun the flute gives you a better grip and an index on where the cylinders are. When you have to reload quick it may be dark, you're motor functions are impaired and you may be sweating or bleeding.
On a hunting or range gun an unfluted cylinder is fine. Not too many instances where a combat reload is needed.