Who will win? The movie studios. Easily accessible pay per use movies will net studios huge profits because it makes the content as easy as cableTV.
It matters little what device plays the content back. They are a dime a dozen and will only get cheaper. Netflix is even opening up their API to other devices like Netgear's HD player and SageTV's HD extender. Each service will have it's pluses and minuses on video quality, audio quality, download speed, etc, but it will come down to whichever box you have in your home already. If you have a PS3, you will likely use Sony's service. If you have a Xbox, you will use Microsoft's service. Some players may even be able to play content from multiple providers. As long as the movie studios get their act together and decide on some firm rules for download, and distribute that to everyone, you will find that anyone can get in the market. I would even venture to guess that niche players will even get in on the service to offer higher picture quality and audio than the mainstream players.