60Sn/40Pb will melt at a lower temperature than 40Sn/60Pb. This may be important if you are soldering heat sensitive components, or on circuit boards.
The lowest melting point available in Sn/Pb solder is 63/37. This is the "eutectic" alloy, which goes directly from solid to liquid, with little to no "plastic" mushy phase. This alloy is widely used in electronics work. It melts at about 360 degrees F.
Most important for electrical work is a non-corrosive flux core. You want a rosin flux, not an acid flux as is used for sheet metal or plumbing work.