Forgotten Realms vs. Eberron? It's not really a quality issue. It's more a taste issue.
With the Forgotten Realms you have an established setting with a mountain of material to draw from. Not only do you have all the various 3rd edition materials, but plenty of material from the 1st and 2nd editions to draw from. It also has the widest variety of novel support of all the D&D settings.
The campaign has an overall very classic high fantasy feel. There are quite a number of very high level NPCs floating around the campaign world, which turns off some people. It's not necessary to have the players interact with them, but sometimes you might have to reach to explain why some of them aren't dealing with a specific problem.
This also tends to mean that it will take a long time before characters become high enough level to become "movers and shakers" in the campaign world (at least from a power perspective, the right player could be one as a low level character, as always).
Eberron, on the other hand is a new setting. You get the advantage of getting in close to the ground floor of setting. A lot hasn't been written about the setting (although Keith Baker, the creator, isn't shy about sharing his view of the world on the official forums), so you have more ability to fill-in-the-blanks without worrying about any existing material contradicting you.
The flavor of Eberron might remind you of Europe after WWI. A very major and long-lasting war has ended, but the tensions are still there. There is a feeling that the war could break out again if someone does the wrong thing.
It also has a very pulpish and noir feel at times. That's the so called "magic tech" that some players will complain about. The Lightning Rail (basically trains) and Airships (blimps) help capture the feel of the pulps. Think Indiana Jones, the Shadow, the Maltese Falcon, Casablanca and the like. Any of these could easily have Eberron campaigns based on their flavor.
Magic is more common in Eberron that other campaign settings. However, it's a much lower level magic. High level magic is very, very rare and usually in the hands of the enemies.
Indeed, NPCs are much lower level that in other campaigns. Very few NPCs over over 10th level. Also, NPCs are very likely to have NPC classes. In other campaigns an officer in an army is likely a medium level fighter, but in Eberron he probably is a warrior with perhaps a level or two of fighter (if he's a heroic character of his own right).
Because of this the players can quickly become "movers and shakers" in the campaign world. Eberron is a risky choice for those who count on high level NPCs to keep the players in line.
Personally, I prefer Eberron. Forgotten Realms never really captured my imagination. If I was going for a more traditional D&D setting I'd likely use either Birthright (if I wanted a political campaign) or Greyhawk.