The Amazon On-Demand service is different than Netflixs "Watch Instantly" service as it doesn't let you watch unlimited movies yet allows you to rent or buy them.
The highlight of this new service is the selection is almost quadruple that of Netflix’s “Watch Instantly” selection. You get more than 40,000 choices from Amazon, including the ability to rent or buy the same day the DVDs are released and day-after TV episode air for the first time.
Oh yeah, don't worry about filling up the memory to quickly as Amazon allows you to also keep and maintain your entire selection on their space.