You will need to explore both benefits and drawbacks from working with an independent testing team, since painting a picture composed only of the positive things would be wrong (and most importantly, it will make you look inexperienced).
I am assuming that by independent you mean by not belonging to the Development Organization, either by taking an outsourced team or by creating a testing organization that does not even report to the VP or Director of R&D.
1. Not biased and/or afraid to say the truth as it sees it, regardless if it is good or bad.
2. Professionalism. If an organization's purpose is to test, then it will do all in its power in order to make this in the most professional way, this will be translated into the more professional testers, up-to-date tools and methodologies, and maybe most importantly an order of priorities that places testing on top of most other activities and operations.
3. If the same group provides services to multiple projects and organizations it will bring with it more experience from having seen more things.
4. Related to the previous one, a group of people with more experience is usually able to work faster since it has streamlined many practices in order to waste less time and reach its objectives faster.
Now take a look at the glass half empty - this is no less important since once you understand your weaknesses you will be able to start reinforcing these points:
1. An outsider never gets access to all information, especially and most importantly the informal information that is essential to navigating a project successfully.
2. The need to sell your next project. Since one of your objectives is to get a happy customer that will also request you for your next project you will always try to please him. This may create a conflict with the first point in the "pros" list.
3. Initial set-up time. In order to be effective you will need to understand the product, methodology, customer environment, and all other things that will influence your project and the way you approach your testing. This will inevitably create an initial project overhead.
I tried describing my top points, but this a very complex issue that can be discussed for month on end.
Sorry if it got a bit long [img]/images/graemlins/blush.gif[/img]|