I Need Help Picking A Pair of Cans!

     I own the HD428s and they are pretty decent headphones, nothing spectacular in my eyes....ears haha, but that's probably because my Ultrasones are so much better that they make the HD428 look like crap. Don't get me wrong, they still sound very good, just not as good as my Ultrasones.  Looking at the Sony's though is cool. They have a better frequency response than the HD428s and they have a longer cord that LOOKS to have a screw-on 1/4 adapter. The Sony's can also fold which the HD428s can not do. Personally the Sony's, to me, look pretty ugly, but if they give the better sound, which I think they will, I wouldn't care.

     The Sony's will not be able to be driven as well as the Sennheisers because of its higher impedance rating. A 9' cord too could be more trouble than it's worth since you said you'll be using this with your iPod on the go. 

     Given your situation, taste, and budget, I would choose the Sennheiser HD428.

See if you can try them both out first obviously. The impedance  must also be considered with the sensitivity factor of the drivers. I can drive 400 ohm perfectly off my Cowon depending upon the can's sensitivity for example, but not sure what the iPod's internal amping puts out. I read in here the Sony are very sensitive as well as built very well.   Might be fine for you.  Heck as with anything amping should improve them you could always grab a cheap ( cost-wise ) and relatively small form factor/slim Fiio portable amp for what $10 or so I think they are now . Then you could use it in the future and not have to worry so much about sensitivity and limiting your choices on future purchases.

Get the 7506(or if you want to save a bit, get the V6 which is almost exactly the same). This is a professional headphone. The comparable Sennheiser headphone is the HD280 pro. If you want more midrange detail, then get the Sony V900HD(aka 7509) or the Sennheiser HD380 pro.