That depends on your situation and what you want to do. If you want a pre-built store that can be set up with specifying a few variables for your system, PowerStore will be a good solution. You can make modifications to the look and feel fo the PowerStore after setting it up. You can also change the payment gateway using eCart if you don't like the one provided in PowerStore.
If you already have a store set up with eCart, you should probably stick with that if it is what you are looking for. If you don't have a cart setup already and you want to completely customize and build your cart based on your own specifications, eCart is useful.
Its really up to you on what you are looking for and where you are now.