I think that 4e is the better system for Eberron, and if your group is already happy with it, you should stick to that. The 4e campaign guide does an excellent job of setting up antagonists and suggesting adventure ideas.
The 3.5 Eberron books are an excellent value. Many of them are mostly fluff, which transfers between editions with only rare issues. I will rank them below based on the utility I think a typical 4e GM would get out of them.
I highly recommend Five Nations, as it gives a good overview of the different nations, which is useful no matter where you plan to be. 5 out of 5
The Dragonmarked book is useful if you have a character that is a member of a dragonmarked house or if you want the dragonmarks to feature prominently in your campaign. 4 out of 5
Races of Eberron showcases the racial cultures of the Eberron races, and is particularly useful if you have characters of those races. It does a good job of fleshing them out and has a lot of ideas you can base an adventure on (shifter contests and sports come to mind). 4 out of 5
Faiths of Eberron goes into more detail on the various religions of Eberron. It is useful to flesh out potential allies and enemies, with information on the philosophies of the faiths, holy days, and the like. 4 out of 5
The Sharn book is also excellent, as it is a central hub and most Eberron campaigns will pass through there. If you plan to have a significant portion of your campaign set there, it is a must have. 3 out of 5
The 3e Campaign Setting has a lot of overlap with the 4e Campaign Guide, but can still be a worthwhile purchase, as it has a different focus in some areas, and not all of the information is duplicated. It also has some useful mechanical/flavor ideas that are not in 4e could be ported over with some work, such as the Siberys Dragonmarks and tiered dragonmarks. 3 out of 5
The Forge of War has useful information if your campaign is going to involve the Last War. It is useful to think about how the war affects modern life, as many people consider the current peace to be only temporary. Less useful if you want to keep the war in the background, a must have if you want to bring it to the forefront. 3 out of 5
The Player's Guide to Eberron is good, has a lot of interesting trivia, and is worth picking up, but I would not put it at the top of the list. 3 out of 5
Explorer's Handbook has useful information on exploring Eberron, travelling, and the like. It is versatile and can be useful for plot hooks, but does not add as much to the game as some others. It has more utility if you plan a campaign heavy on travelling around the world. 2 out of 5
Secrets of Xen'drik is useful if you plan to use the continent of Xen'drik in your campaign. Since it is a great area for pulp action, I think it is fairly useful, although you could do without it. 2 out of 5
Dragons of Eberron gives some more specific information on the Eberron dragons. It has some useful information, and if your campaign focuses on the Draconic Prophecy and will include multiple dragons, then you will want to pick it up. Otherwise I wouldn't worry about it. 2 out of 5
Secrets of Sarlona will be useful if the Kalashtar, Riedrans and Dreaming Dark feature prominently in your game. Also useful as an alternative setting in Eberron if you want to go that route. 2 out of 5
City of Stormreach is handy if you plan to use Xen'drik heavily in your campaign. Even if the PCs only go there once, it can be useful. However, you can get by without it. 1 out of 5
I would avoid Magic of Eberron, as it was more crunch heavy, and I was not particularly impressed by it even in 3.5. 1 out of 5