When choosing an aquarium you generally have two options to choose from; a tank and optional hood or a complete aquarium system. There are pros and cons to both options so you will need to consider both options, weigh the pros and cons and then choose what is best for you.
Buying a tank and hood outside of a kit allows you to choose the various parts that you want to get with the tank. It gives you complete control over what type of filter and heater you are going to buy for your tank. However, buying all the parts individually can get pretty expensive. Aquarium kits save you money by including all the parts you need to set up your tank and get it ready for your fish.
Complete kits not only include all the parts needed to set up a functional tank, they often include custom, built-n filtration systems and many of the built in systems in the lager systems are superior to the filters that are sold separate from the kits. The higher quality complete aquarium systems also usually come with a stand that is specifically designed for that tank. This eliminates the need to search for a stand that matches the dimensions of the tank and will hold the weight of the tank when it is full.
I have bought tanks both as kits and as individual aquariums. My recommendation is to choose based on your budget and the space available for your aquarium. If your budget is low you can buy a starter kit from a major chain pet store and save some money. If you have a larger budget but space is tight you can either buy a small aquarium system or buy your parts separate from your tank so you can get better quality filtration for your tank. If you have the space and the budget, you shouldn't consider anything but a full sized complete aquarium system.
Read my review of the Solana XL Complete Aquarium System