When you want to go up or down quickly then use level change. When you want more control when going up or down slowly then use v/s.
If you use a high rate of climb in v/s, then the autothrottle will go to max climb thrust, but the speed will decay. So much so that you can approach the stall. You can also do a similar thing when going down with too high a rate of descent and end up approaching Vmo.
ATB...Cough