for bigger projects i recommend eclipse, for small projects/short sketches its better to use the processing IDE.


the biggest advantages in eclipse are: (eclipse is constantly assisting while coding)

• Code Completion, instead of digging through documentation you should be able to tab your way through methods and save yourself a lot of writing

• Refactoring, Global Find and Replace is no replacement for good refactoring support, that starts with renaming functions, variables, classes, ... and ends with some of the functionality that you can find in current Java IDE (IntelliJ for example)

• Syntax Checking, helping you out with writing correct code while you type

(copy&paste: http://stackoverflow.com/questions/136056/ide-or-text-editor/139310#139310)

these are the things i really missed in the processing-ide after working with eclipse for a while.
but sometimes i switch back to processing-ide for small sketch-studies, since making a new project is much faster in processing, than in eclipse.

Excellent overview above...
You can add:
- Superb debugging tool, with step by step, break points, variable inspection, etc.
- Excellent navigation capability (click on a method use, go to its definition; find where a method is used and the hierarchy of calls leading to it; etc.).

We should also present some downsides of using Eclipse:
- It is big and quite complex. Processing IDE simplifies lot of things, like adding a library.
- Some things hidden by Processing have to be handled manually, and some things must be changed from standard sketches:
  * You have to do the base imports yourself; now Eclipse makes that dead simple, so that's  not a major issue;
  * You have to remember to replace color type by int, add f at the end of float literals, don't use int() but (int), and so on.

I would say the Eclipse is mostly for the advanced Processing developer, starting to have big sketches.
Personally, I code Processing for years, but doing mostly quite small sketches, I haven't felt the need to use Eclipse for that. Despite the fact I use it daily at work, for a very large project.
I don't use Eclipse for Processing, but I often use an external editor, PDE's one is... lacking.

I'm using Eclipse too, especially for coding for Android. Not only that you can run and debug Android apps on your physical device, you can also rather use an Android-x86 installation in a virtual box than the standard emulator from the SDK. Instead of emulating an ARM CPU, you can simply take an Android version that is compiled for your PC hardware. This hosted solution is so fast that you can "emulate" your sketches in real-time where the original emulator only gives a slideshow. Just follow these steps to setup :  http://androidspin.com/2011/01/24/howto-install-android-x86-2-2-in-virtualbox/ and then connect it to the Android Debug Bridge.