Should I use the exercise/equipment that uses more core?

"Pull ups are better than lat pull downs, because pull ups make you use your core more."

"Dumbbells are better than barbells are better than machines because they use your abs/stabilisers more."

"Fitballs are great for abs."

If you've been around gyms and fitness communities long enough, you've heard the above statements (or words to that effect) before. Everything is better if it involves your core more. Although for some reason everyone draws a line somewhere when they make these claims. For one person, dumbbells are great because they use the core more, but one-armed/one-legged exercises (which use the core even more) are just silly fluff. For another person, that line is drawn with fitballs and bosus. For another person, they'll be doing cartwheels and triple jumps on fitballs while pressing a kettlbell overhead with one arm before they'll even consider that they might not be doing the best thing possible.

There are a lot of things to consider before deciding whether an exercise is brilliant or stpid or something in between, but the main question is: "To what end am I doing this?"

If you're doing an exercise that's just for fun, and has no major purpose beyond that, then so long as you don't hurt yourself (or, at the very least, so long as I'm not liable to be sued if you do hurt yourself), who am I to say that it's not a good exercise, or that there are better exercises for you to be doing?

Of course, if you're doing an exercise to achieve a specific result -- and any exercises you do that go under the "serious training" aspect of your routine had better be there for a good reason -- then it's simply a matter of whether or not you're doing the best exercise for the result you're after. So, when someone says you should do pull ups instead of lat pull downs because pull ups use your abs more (which may not be correct in the first place), if the reason for doing that movement is to strengthen your lats, lower traps and rear delts, then increased core activation in one exercise over another is a minor selling point at best, if not irrelevant. More over, if the selling point is that exercise A uses your core/stabilisers/whatevers more than exercise B, but exercise A doesn't achieve the results that exercise B was chosen for, then exercise B is superior.

Continuing with the example of pull ups vs lat pull downs, I've noticed that -- especially to beginners at strength training -- it's typically easier to pull with your back when doing lat pull downs than when doing pull ups, simply because your body isn't moving around much and you can use a light load to focus on technique, rather than heaving with everything you've got just to get out 1 or 2 reps. Using myself as an example, it's literally taken me years to learn to pull with my back for pull ups, whereas lat pull downs didn't take very long at all to figure out once I put my ego aside and stopped trying to move as much weight as possible. So, if I were doing a vertical pull for my back muscles, for a large portion of my lifting history I would have been better off using lat pull downs than pull ups, regardless of what core activation may have occurred one way or the other.

So again, what is the reason why you're doing the exercise in the first place?

Of course, if you have two exercises to choose from, and both do the intended job just as effectively as each other, but one has additional benefits, then it may be wise to choose the one with more benefits, provided it doesn't somehow interfere with the rest of the program or unbalance an otherwise balanced routine. But when an exercise takes you away from the purpose of that part of your program for some other benefit (especially if it's a benefit that's being dealt with elsewhere), then the other benefit may not be worth it.