Very few differences experienced in 1.2 vs 1.3 SDK. Stuff made for 1.2 works in 1.3, and only new stuff in 1.3 would not be backwards compatible to 1.2 (SAPUI5 sap.m components)
1.2 -> 1.3 = not a problem
1.3 -> 1.2 = can be a problem, depending on component.