The physical devices are yours; you can be certain your data never leaves the building. With rented, cloud-based mobile devices you'll likely need to re-install every day and possibly wipe at the end of every session, or risk information leakage.
Also, the UI experience with a mouse is not the same as with a physical device - you may miss subtle usability issues.
Generally speaking you can get better coverage of more devices for less money with the rental model, so what makes sense for you probably depends on your budget, timeline, what you need to support, your userbase, how new and fancy your UI is, etc.
