光圈大 = more light goes into your lens = allow you to use faster shutter under the same lighting condition.
IS = help you to stabilize your photos to compensate your hand-shake (more useful for tele photos, say over 100mm+)
If your kids (not you) run around, 光圈大 is more useful because your shutter can be faster.
"IS" does not help to "freeze" your kids when they run around. It ONLY helps you compensate your hand-shake (say you don't use a tripod).
光圈大 + "IS" is nice!! So you can walk around to take photos of your running kids.