it also depends on whether you want a slice in the final assembly or whether you just want to split a single sketch into two sketches. if all you want is a slice in the final assembly, just draw a new subtractive sketch along the plane that you want to split the object by. if you want to split the sketch, you can either create two new sketches which have the two pieces you want, or you can use the split or cut command. if you think about it, the cut command is just a shortcut to re-creating the sketch as two separate sketches. (in programming languages, this is called 'syntactic sugar')