You can buy HD or SD. You choose, not itunes. Buy whichever you want. When you buy HD, the SD version comes with it.
How many times can I view the movie?
Once you start watching your movie, you can view the rented movie as many times as you wish within the 24-hour (or 48-hour) window."
Are rented movies available in HD?
Yes, select movies are available for rental in HD. You can rent and view HD movies on your computer, iPhone 4, iPad, iPod touch (4th generation), or Apple TV."
Can I play my rental on more than one device?
- If you download a rented movie on your computer: You can transfer it to a device such as your Apple TV, iPhone, iPad, or iPod if it’s a standard-definition film (movies in HD can only be watched on your computer, iPad, iPhone 4, iPod touch (4th generation), or Apple TV). Once you move the movie from your computer to a device, the movie will disappear from your computer's iTunes library. You can move the movie between devices as many times as you wish during the rental period, but the movie can only exist on one device at a time.
- If you download a rented movie on your iPhone 4, iPad, iPod touch (4th generation), or Apple TV: It is not transferable to any other device or computer."