For a lot of people, no, and it'll mostly be about how much it's worth it to get the same amount of protein with 15-20% fewer overall calories—though the flip side of that is that you will get more protein per pound of iso so you have to take that into account if your goal is Xg/serving. If you struggle with lactose/dairy, though, it can be a pretty big deal and definitely worth the price difference.
Also, for MyProtein, as somebody who does have major issues with lactose, the most I've paid for their iso was $7.09/lb (which is still better than the MuscleTech deal here adjusted for protein content) and the best is $5.09/lb. The current discount has it at $6.50/lb which would definitely make it worth a bag if I was running low. Impact's at $5.18/lb which adjusted for protein content is... around $5.95? Something like that.