First you must know what you are into. What type of photography are you more interested in. Only then, will you know what lens suits you better. Plus what is your previous camera? If it was an entry level Canon DSLR, I would expect you already have some Canon lenses, so u should know the IQ of kit lens and whether it was acceptable for you.
There are some other 3rd party lens that have fabulous reviews too... such as Tamron 17-50mm non VC. You could always check out reviews online for the IQ of these lenses to have a rough idea of how it would perform (one good site is http://www.the-digital-picture.com/Reviews/Comparison-Tools.aspx). Then I would suggest that you go down the store and try them out and see if it suits you. Take care of the weight of the lens too.
Have you tried the 18-55mm? I think it is a very good lens in relation to its price.
Of course, what 3rd party lens are you looking at, and maybe you can also let us know your budget and the type of photography you would be using it for, so we may better advise you.
Buy what you know and what you need. Have you defined your shooting needs already? Have you found shortfalls in your current lens or camera? If you can't really say what you need you better stick to what you have. Upgrading skills often results in better pictures, upgrading body rarely does.