Oops I goofed! I updated my post and fixed the links.
If you are going to be using them for portable use, I would get the coiled because it is more manageable and can be stuffed into your pocket. The straight cable is over 10ft long! So that makes it much harder to manage when you are mobile. Also, another thing you should note is the ATH M50 tip. The tip is wide and will not connect/reach the iphone jack, you will have to use an adapter.
Here is a pic so you can see what I mean:
You can just buy a cheap iphone splitter on ebay which does the same thing as an adapter and also gives you the bonus of sharing another output if you'd like. That is what I use for my iphone. Here is a link, less than $1 shipped, but it takes a long time to ship because it origins from China:
http://www.ebay.com/itm/Headphone-earphone-splitter-audio-cable-3-5mm-double-jack-iPod-iPhone-4-4S-iPad2-/190879574454
If you don't want to wait, you can order from a US seller for about $2:
http://www.ebay.com/itm/3-5mm-Audio-Headphone-Splitter-Cable-Adapter-For-iPhone-MP3-MP4-/191205659682
Also, if you think about it, for the price of the newer ATH M50x, you could get the original ATH M50 AND a refurbished Logitech UE6000 with the money you save. You could also get an amp instead or just pocket the savings. Lots of options