USUALLY you can get a cheaper price on a kit......but you also get what you pay for. I have read posts just reciently here about people who have purchased a kit, just to find out the heater didn't work....
I MUCH prefer buying a tank, and getting JUST what I want in the type of filter, heater, and other equipment.
The first 55 gallon tank I got had a hood with light, and shortly after I purchased it, I found out the lights weren't enough to keep any but the lowest light plants.
There are many people here that can give good advice on JUST what to buy, and the least expensive places to shop. Often you can get really good prices and quality from on line.
You can also get good deals from e-bay or craigs list. Start with the biggest size tank you can afford, (maybe from craigs list), and customize your other equipment to fit what you want to do with your tank.
