I am trying to replace my Sony MDR-XD400 headhpones.
I like these headphones because they let ambient sounds in, which is helpful when using them with a computer. It appears this is due to them being Supra-aural headphones. However I also see that Open-back headphones can serve this same purpose.
What type would be better for my situation?
It depends, quite honestly. For your purpose, it sounds like open back circumaural may just be more comfortable, a la the DT880 or 990s (880s are semi) and give you natural-sounding bleed through (source), since supra-aural might smoosh your ear and as a result alter the phase qualities of what you hear inside and out. For an example of what I mean, try A/Bing with your hand covering an ear while listening to something like an HVAC (and ever so slightly altering between how much you smoosh your ears with your hand), and also try cupping your ear with your hand (causes a more telescopic effect reminiscent of how a Telinga parabolic might work). It drives home the point that our ears and their relationship to airflow is as important as the sound source delivery method.
Again it depends on your needs. The 'textbook' reason for open-back circumaural in music and post sound professionals is that it's closest thing you can get to working on monitors (allowing air to propagate and develop the frequencies properly versus a vacuum) if we aren't able to run monitors. As a result, the response tends to be flatter (than say the MDR 7506) and offer a slightly more reliable translation (i say slightly since it never will replace working on monitors) - at the cost of letting background noisefloor bleed through.
That all being said, for your needs they may actual serve the purpose you need.
Just my two cents. Hope it helps!
