If I have the option, I’ll use them both. I only really have that option with a few clients that I train in a gym, since I work with most of my clients at their homes. I’ll use a stability ball for leg curls also – I’m not a big fan of those, but they’re fine for my older clients (which is most of them!). I understand that isolation exercises such as this are “out of vogue” these days but I’m not buying it. If you’re wondering if you should be including them at all, I would say yes.