Wayne,
As far as the three grades, 60 is the most common, so availibility shouldn't be an issue. The carbon equivalent of 615 is not controlled and must be determined through the chemical analysis shown on the actual MTR. The carbon equivalent usually falls between 0.6 and 0.7, so preheat is almost always required for welding. After calculating the C.E. of the rebar, refer to the Minimum Preheat and Interpass Temperature Table in D1.4 for preheat requirements, which is based on the C.E.and rebar size. The formula used to calculate the C.E. is also shown in D1.4.