The choice of development environment is not an easy one. I will try to give some advice that goes beyond technical specifications.
The discussion of which IDE is the best easily turns into a holy war, much like the discussion of where to put the braces in your code. With braces, I don’t go with the solution I think is the best, I go with the solution that works the best, and I handle the choice of IDE the same way.
For Java, IntelliJ’s IDEA is the best IDE on the market, IMHO. Unfortunately, it costs some money. Not a lot, not even near the cost of switching from one IDE to the other, but enough to require purchase of software licenses, which in some companies is an operation of considerable size and complexity. Often it just won’t happened without a battle. You have to choose a free IDE to be sure you can introduce it without financial friction.
As a software consultant, your own company could buy the license, and then you use it wherever. While this might sound good, as being an agile guy you might want to pair-program from time to time, and if you use a uncommon tool, it might be hard to work effectively together with someone not used to that tool, and you easily get a “driver-passenger” situation. The IDE-of-choice should also be the most commonly used IDE to improve chances of good pairing.
If you do the development internally at you company, you and your developers are in a pleasant situation. You can and should choose the best IDE, free or not.
If, however, you have contractors, the balance shifts, and you should go with a paid-for IDE only if the contractors are few or they are on long term contracts. Otherwise you should go with the commonly used and free IDE, to improve contractor chances of familiarity and hence productivity.
In conclusion, as a contractor or consultant you should be used to work with the most commonly used, free IDE. As a buyer of such services you should probably do the same.
E.g. for Java, I believe that Eclipse is the IDE that works the best, and that without being the best. It is free, commonly used and definitely good enough to be productive with, and nowhere near worthy of a holy war.