That EF-M 18-55mm was better than the EF-S 18-55mm MkIII then Canon realised their "mistake" and stopped making them and replaced it with the cheap 15-45mm. I'm looking to Tamron to fill in the gap with the best EF-M standard zoom with f/2.8 at the widest end.
It depends on what you are shooting. The 15-45 is smaller, much lighter (hence a better fit for the M100/200), and also wider and sharper at the wide end. The 18-55 is bigger, heavier, longer and sharper at the long end. 15-55, or better yet, 15-85 would be nice, but in both cases, we are talking a considerably bigger lens. The EF-S 15-85 is a lovely lens, but by M standards, it is huge. Other than the 22mm pancake, the 15-45 is the most portable of all. In any case, keep watching. The EF-S 18-55 has more versions than Carter has little liver pills (if you are young, look it up), so yet another M kit lens is highly likely, if not inevitable.