In a blog post announcing the move, Hulu CEO Jason Kilar doesn’t explain the rationale for the price cut, which comes less than two weeks after the service’s formal launch.* So feel free to make your own guess.

But the announcement highlights the core tension facing Hulu, which is jointly owned by Providence Equity, GE’s NBC, Disney’s ABC and News Corp.’s Fox: The best move for the online service is often at odds with its owners’ strategies for their businesses.

In this case, Kilar has been pushing for a price cut while the networks have been trying to effectively establish higher prices for their programming, in the form of new fees they’re demanding from cable providers (see: Fox. v. Cablevision, etc).

The new $7.99 per month price should make Hulu Plus more competitive with Netflix, which offers DVD rentals along with all-you-can-eat streaming for $8.99. So will another move to add more programming from NBC Universal, which Netflix got access to in September.

Hulu Plus is supposed to appeal to customers who want access to a deeper catalog than Hulu’s free service, and who want to watch the shows via devices like Apple’s iPad and Microsoft’s Xbox 360–all of which support Netflix.

Hulu says it will provide $2 credits to existing Hulu Plus subscribers who signed up during the trial phase this summer, and is also offering a few other promotional bells and whistles to push the service. From Kilar’s post:

• One free week trials for all new subscribers. In addition, current subscribers who joined during the preview period will receive a credit for one week of Hulu Plus toward their next month’s subscription.
• Two free weeks of Hulu Plus for both current subscribers and friends they invite through our referral program. Subscribers can learn more by clicking on the “Referrals” tab on their Hulu profile page.
• 11 weeks ($20 worth) of free Hulu Plus with the purchase of a Sony BRAVIA connected TV or Blu-ray player through January 31, 2011. (See sony.com/huluplus.)
• One free month of Hulu Plus with the purchase of a Roku device through December 15, 2010. (See roku.com/hulu.)

*Hulu points out that Hulu Plus only formally came out of beta today; two weeks ago it dropped the invite-only requirement for the service.

Amazon (AMZN) launched a new service called Amazon Video on Demand, which allows users, for a fee, to watch any of 40,000 TV episodes or movies, in real time, on their Windows and Macintosh computers, and on specially equipped Sony Bravia TV sets. This service replaces an earlier Amazon video offering called Unbox.

I have been testing Amazon Video on Demand and I found it worked well, although it has some limitations. The user interface is clean and smart, the quality is good if you have a fast Internet connection, and there are some clever features. On the downside, it works poorly with the slow connections typical in places like hotels. And there are some studio-imposed limits on what content is offered and how you can view it.

To date, there have been three major models for legally getting TV shows and movies from the Web.

In one, best represented by Apple’s (AAPL) widely used iTunes store, the TV shows and movies are ad-free, but you pay for each. All of this content is downloaded to your Windows or Macintosh computer or your Apple TV set-top box for later viewing, even when you’re not connected to the Internet — though on Apple TV you can start watching while the material is being downloaded.

A second model, best represented by the studio-owned Hulu.com, presents movies and TV shows on a Windows or Macintosh personal computer free of charge, but requires you to watch commercials that can’t be skipped. The TV shows and movies you view on Hulu are “streamed” rather than downloaded, meaning they are meant to be viewed immediately, in real time, rather than stored for viewing when you’re not connected to the Internet.

The third online model is best represented by Netflix (NFLX), the popular DVD distributor. It offers a “Watch Instantly” streaming option on Windows PCs or on TVs equipped with a special set-top box, for a small portion of its large catalog of TV shows and movies. These videos are ad-free and don’t carry an individual charge, but require a monthly subscription fee.

The new Amazon Video on Demand service is a hybrid of these others. As on iTunes, the TV shows and movies it offers are ad-free and purchased individually, don’t require a subscription, and work on both Windows and the Mac, plus on one type of set-top box. In Amazon’s case, that’s Sony’s Bravia Internet Link, a $299 device that works only with Sony (SNE) TVs.

But, like Hulu and Netflix and unlike Amazon’s older Unbox service, the new Amazon Video on Demand service offers videos via real-time streaming. In many cases, it also allows downloading, iTunes-style, to Windows PCs (but not Macs) and to TiVo devices attached to a TV. The videos can’t be streamed in real time using a TiVo (TIVO).

Amazon’s streaming videos are viewed in any of the major Web browsers and don’t require any special software. I tested the new service on both Windows PCs and Macs, and on a Sony Bravia TV equipped with the $299 adapter box. In my tests, Amazon’s videos looked quite good over a fast Internet connection. However, on a typically lousy hotel Internet connection, the movies were often grainy and kept stuttering.

The new service doesn’t yet offer videos in high definition, something Apple just announced this week it is adding for some TV shows. Amazon says it is working on HD.

Amazon’s user interface for the new service is very nice. All of your purchased videos are available in a library stored on the company’s servers, so you can easily watch them again and again if they are purchased, or resume a partly watched rental. And the service remembers where you stopped watching a video and resumes it at that point, even if you started it on, say, a Dell (DELL), and resumed it on a Mac.

On the downside, the new Amazon service isn’t always simple or consistent. For instance, you can watch only two videos at a time, and not all titles can be either streamed and downloaded, or be either purchased or rented. Most rentals last 24 hours, but some differ.

Selection was OK, but not great. Because of studio policies, many current and recent movie hits aren’t available. There are gaps as well in the TV selection. For example, while iTunes offers the current second season of the excellent “Mad Men” series, Amazon has only the first season.

Prices also can be confusing. Amazon rents most movies for $3.99 and sells them for between $9.99 and $14.99. TV shows generally cost $1.99. But some titles carry different prices, albeit these are often lower.

All in all, Amazon Video on Demand is a good service for people who prefer paying for ad-free TV shows and movies, and is another strong step in the Internet’s rising competition with traditional TV.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.