Ok, here is another breakdown of the services and how they work....
First, you have UltraViolet, which essentially is what format we are converting our DVDs and Blu-Rays to when using the disc to digital service. You get to choose whether you want SD or HD/HDX but ultimately, no matter which you choose, it's still UltraViolet digital format.
Second, you have your digital streaming and/or download providers who allow you to link your UltraViolet account aka your UV cloud, to their program, app, etc to allow full access (most of the time) to your collection. These companies include but are not limited to: VUDU [Walmart], Flixster, Target Ticket, & Cinema Now [Best Buy]. For the U.S., these are the main providers who allow this kind of access to your UV cloud at the current time and personally, I prefer a combination of VUDU and Flixster, even though I have not used much of Target Ticket or Cinema Now, they don't seem to offer as much (will elaborate soon) as the before mentioned 2.
Lastly, you have what each company offers to the table. Flixster offers full support for PC and Apple, not to mention support for iOS, Android, and I believe Windows phones as well. They also offer Xbox and Playstation support for their consoles. I stream movies and shows from my Samsung Galaxy S (yes, the original. I'm cheap and if it's not broke, don't fix it kinda guy) almost on a daily basis while bored at work. The beauty of their service is they allow you (and for quite some time now) to download your videos to your device so you can watch them while offline. This is perfect for times when you know you might be going into areas with bad service for prolonged periods of time so you can download a few titles to watch while in the "dead zone," so to speak. VUDU also offers something similar through their VUDU To Go application but I have not used it yet nor do I know if they have mobile device and/or tablet app support. Other members, who may or may not know the EXACT details on their services ~cough~umbry~cough~, would be able to inform you better of the things they offer.