I found the taste to be very different to be honest. Both are nice but the pre-mixed drink is my favourite but you can improve flav of the powdered drinks with a little creativity.
As a long time JJ consumer I learned to mix them up with other ingredients. In the chocolate shake I often put a spoon of Nesquick in to give them extra flavor. Same with Banana, I add some extra honey and a banana and blend it all in a blender for a moment before I chill them in the fridge.
In all cases I fill my shaker with crushed ice first to prevent any lumps from sticking to the bottom and the melting ice does change the texture as with mixing drinks for cocktails. The crushed ice gives it a better texture when shaking.
I have tried to make the vanilla taste the same as the pre-mixed vanilla by adding a bit of honey and custard powder but failed at that even though the vanilla shake does taste very nice with a dash of custard. The strawberry shake I often mix with frozen berries and then put them in the blender for 30 seconds. It also changes to color a bit and makes it look more appealing.
Another alternative is to use milk instead of water to mix the shakes. Even skimmed milk gives all the shakes a super creamy taste and think it really improves the overall experience.