Horns are more open and airy, better articulation and dispersion. So they do sound different. I prefer them, some do not. Totally your choice as to what you like.

Your links are two different types. The horn loaded speaker will be easy to drive and have a nice wide dispersion. The second is a MTM design. It has a controlled vertical dispersion which reduces reflections off the floor and ceiling and keeps the even dispersion of a single driver in the horizontal plane. Also, with the tweeter directly between the midrange drivers, the sound appears to come from a single point source.

 

What is the difference between a Horn loaded tweeter and a regular tweeter? Is there a sound difference? Is one better than the other?

One mus wonder what you consider "regular". Cone? Dome? Inverted dome? AMT? Ribbon? Plasma (look it up, there is such a thing).

Horn loading was a way to get a driver to better couple with the air. In short: it gets louder (all else being equal). The horn also acts as a waveguide and affects dispersion.

http://en.wikipedia.org/wiki/Horn_loudspeaker

Like most speaker technologies, the implementation matters more than the technology. Because the drivers are smaller, you will tend towards better (crud: the word just fled my brain... speeds of transitions and impulse) than, say, a cone; though perhaps not as good as a ribbon.

In the real world, I've found horn-loaded tweeters more likely to sound "harsh"; but that's both preference and , again, the specific construction; not the technology itself.

Does this style speaker have an advantage over a single woofer?

I am considering getting 6 bookshelf style speakers for a 6.1 system (in our basement) to go with my Onkyo TX-NR709 AVR. If I could get 5 matching bookshelf speakers I would go with a 5.1 system.

If we are comparing 2 of woofer X to 1 of woofer X then the advantage 2 has is that it's +6db. As such, it can either go louder (by 6db) or lower (by moving the -3db point farther down the frequency chart.

There are, however, some potential problems with off-axis performance of paired drivers.

In the end: you need to listen for yourself in your room. Most of what non-listening evaluations can do is rule out speakers (this one's not loud enough, that one has horrible off-axis performance, etc)

 

Horn loaded tweeters increase the efficiency and can give a speaker better dynamic abilities but another great thing about them is their ability to control the directivity of the sound. This helps a lot in imaging due to room effects having less influence. The level of directivity varies from design to design though.

Bad designs can sound harsh. Good designs are smooth airy and clear.