The differences between the two units are very subtle. Most people, speaking in mass numbers, end up choosing based on the price or the appearance of the units. The core technology between the two units (picture quality, etc) are similar.

The LNxxA750 units do have a few differences.

While the A650 series will play music and pictures off of a thumbdrive, the A750 adds video functionality.

The A750 also adds a Content Library feature, which has games, recipes, exercising/stretching modules, and some other features. There also is some content available online to download and use (Free and Paid is available). It's a first generation idea of putting unique content in the television itself.

Here is the additional content available for free:

The A750 is also DLNA Certified. (Digital Living Network Alliance). As the computer becomes more of a central peripheral to the television, this technology could be useful for some current, and more future applications.

As far as the "Halo" effect, it does occur occasionally. Naturally, I can't guarantee that you won't have it, however, we have put measures into place, such as redesigning the bezel from the factory and offering to replace the issue should you run into that issue.

Either set would be a great set to have.

I hope that helps.