You already have an AKG which you like and the 701 is a very definite improvement. Unless you are very sensitive to what you look like wearing a headset, don't bother with the 702, at +100$ it's only different in color and connection. The character is like for like the same, although some people of course hear a difference. In Europe we're getting the 701 at stores like thomann.de for around 200 euro's. Amount to about 250$ I think.
I have listened to both in a store (not the best condition) and keeping in mind that a "good" headset is more about taste, the immediate thing I noticed was that the Sennheiser sounded like a (very good) headset, and the AKG didn't sound like a headset. In my opinion, the AKG is more natural, wider/deeper sounding and has less "standing wave" issues (as far as they are an issue with a headset ; ). The Sennheiser is a bit dirtier on the bass which will lead most people to think the AKG has no bass at all in a one on one comparison. Since I believe you can train your ears if you have better "input" I would also say the AKG has more headroom for learning.
Speaking of headroom, what is a concern is comfort. I would say that on long sessions, a minute difference in quality and even a minor difference in taste is greatly offset by how invisible the headset is to wear. The leather band on the AKG does prevent the headset from pressing on the sides of your head (to stay stuck) but I have yet to wear it for any extended period of time (same for Sennheiser). I found the band obtrusive thanks to the repeated "large bubble" pattern but might get used to it.
Either way, you won't go wrong with neither, they are fantastic value for their price.