The MDR-7506 is pretty much entry level as far as monitor headphones go. Also, it is not particularly neutral, so I doubt you would find it in many audio post facilities. I personally found the frequencies around 100-200Hz and 1-3KHz a bit hyped. This does not always translate well to your final mix, as you may be inclined to overcompensate by decreasing these frequencies in your mix.
The big question is how much time you will be spending on a regular basis with headphones on your head. Any longer than an hour a day means it plays an important part in your life and, if this is the case, I would recommend listening to at least 4 or 5 different sets within your range of affordability.
Always take along a selection of tracks with which you are very familiar and which represent good examples of the type of material that you expect to be working with. Whatever you do, don't listen to anything provided by the salesperson. The set that reproduces your tracks the closest to your expectation, subjectively of course, will most likely be the best set for you. There are other factors that must be considered, such as comfort and construction, but only you can determine how to weight these in relation to your specific requirements.
I think you should include the Sonys in your audition as a reference point and, at the end of the day you may even end up choosing them depending on your subjective experience. Personally, I have been converted to Ultrasone (you must include a set from their PRO range in your audition), but AT, AKG, Sennheiser, Shure and one or two other companies all make quality monitor headphones in a similar price range.
I used the ATH-M50s for a while and quite liked them tracking, as they have quite a natural sound and allow for extended use with minimal fatigue, although I would not use them for mixing, editing or mastering, as I feel they are a bit lacking in detail. Then again, I do a lot of restoration work, so detail and comfort are the two most important factors for me, as I use my headphones 90% of the time in relation to my near-fields.
In effect, what I am trying to say is that headphones are personal and, if you intend to spend a lot of time with them and use them in a professional capacity, you should not underestimate their importance.