Renting a list means that you will only be able to use the contact information once (and only see the contact information of respondents). You will also likely never actually see the list; in this type of situation it is common to email the list owner and have the message broadcasted to the list.
Purchasing a list is widely considered the best option for many reasons. For one, you will have permanent access to the addresses included therein and databases can be purchased based on criteria (such as location or industry). Purchased lists are a great way to expand the profile of your company in an efficient way, and a really great way to build up your marketing profile quickly. It is far more cost-effective to purchase a list once instead of repeatedly renting the same names every time a new marketing is ready.