The long and short of it is that the Vostro is geared toward business and Inspiron is primarily marketed toward home users. Lots of people prefer the more business-end laptops as they tend to hold up better over time. Dell typically has fairly deep discounts on the 14" Vostro line here and there, which makes it rather attractive.
I bought a Dell business-line, it was a Latitude, and the service is amazing. You get a longer warranty, and it includes on-site service. That means they come to your home, with spare Mobo and keyboard and whatnot, and fix it right in front of your eyes. In my experience, they aim to please the customer, they are not looking to blame the user in order to get out of the warranty.