Mass gainers are, for the most part, just loads of calories. The macros are usually split nicely but there’s no additional nutrition. It’s not the same as eating a healthy meal, but then they’re not meant to be.
Huel on the other hand is nutritionally complete, so a shake is on par with eating a healthy meal of the same calorific value.
I’d say Huel is better. Afterall, if mass gainers didn’t exist you’d eat more food, right? But you wouldn’t eat something that’s just super dense in calories, you’d eat something nutritious. Huel does that as a shake.
I would echo what people have said here. The benefit of a gainer is it’s super convenient to get the additional calories you require to gain weight (calories in calories out). The difference is that they aren’t offering anything in addition to those calories. Also, you asked for differences, one difference is that the gainer you’ve pointed to has about 50g carbs per 400kcal, whereas Huel is about 38g. Huel is just as convenient as a gainer but offers far more nutrition, so would be a better option.
On a personal note, I’ve found gainers to be not particularly filling as they have quite a thin texture. That might be what you’re looking for, I don’t know what this one you’re looking at getting is like, but Huel is filling.