The principle of overload is what makes you stronger:
-It's very VERY important to use enough weight to make your sets somewhat difficult. However many reps you do, by the time you get to the last one, you should not be able to complete more than one or two more.
-Using an unstable device of some sort will compromise overload- you can't lift as much, so you aren't going to get as much benefit. This goes for upper/lower body exercise as well as exercises for "core" muscles- you're best off to work them directly with sit ups (weighted, if necessary), good mornings, ab machines, leg lifts, reverse leg lifts, etc. This will make them strongest.
- When you are just starting out, any exercises done in any number of sets/reps will make you stronger, but to continue to see results, you'll have to increase difficulty in some way/shape/form