If you really need the power of multiple stackable bitmap graphics, then use DrawIt. If that's not what you need, you're probably better off with Sketch. Also for schematic drawings we would recommend Sketch.
Sketch is fully vector-based though. Zooming in in DrawIt shows you its precise pixels whereas zooming in with Sketch produces smooth curves for vectors and text. As such, Sketch can also export to vector formats.
Sketch also excels when you need to create large documents; DrawIt was optimised for smaller documents, say up to 1024x1024. Other features that are unique to Sketch are its Infinite Canvas, Symbols, Multiple Pages and better slices and superior Text support.