It depends how much you use the internet. Its important to also remember that its not just what you physically download yourself, that allowance includes software/windows updates, which are getting bigger in size all the time.
I use the internet a lot, and to be honest would struggle with anything less than unlimited downloading. For example, this month so far I've downloaded 146GB alone, on just my main PC. That doesn't even take into account our Macs, laptops and other computers in the house - which is something else you need to consider if your connection will be shared by others.