Fostex' affordable range of Planar Magnetic headphones has really catered to all preferences in offering an Open Design (T20RP), a Closed Design (T40RP) and a Semi Open Design (T50RP).
To make sense of these different models, it's worth understanding what the different design offers the listener.
Fostex T20RP (Open)
This is an open design which gives the listener more powerful bass with a slightly more 'authoritative sound'. As these are open, it does mean that you'll be able to hear more background noise, and other people are more likely to be able to hear your music.
Fostex T40RP (Closed)
This is exactly the opposite of the T20RP, a closed design. This results in a more even bass response and has a slightly better mid range response because of it. It also gives you much better isolation to the outside world, so probably a bit more ideal for commuting.
Fostex T50RP (Semi-Open)
These seem comparatively bass light to the 20 and 40, however are possibly the most accurate. They seem to offer a nice balance. These do still leak a little bit so possibly best for home listening or in a more isolated cubical. Great for walking around.
Associated Frequency Response Chart