VFS Global is a commercial company, working in partnership with the UK Border Agency, they only process your visa app and collect the money, they have nothing to do with helping you get a visa or making any decisions about granting you a visa, thats done by the embassy only.
while the other company i take it, will help you fill in the forms and give you some advice, and you or them will have to submit your app to VFS anyway..
if your case is pretty simple, i would do it your self and not waste money using any sort of agency
