Hi there.
Comparing the Sennheiser's with the AKG's is like comparing two very different kind of sounds. I had an AKG already a long time ago (the K280 Parabolic - looked wierd) and started to drive it with preamps. It was nice.
Until I changed to the Sennheiser 600 and knew what I was missing. The Senn's had much more 'body' to the music, more punch, more drive, more resolution, you felt like you'd come as close to the music as you could.
That experience even increased when I got myself the Darkvoice 336i (I never thought I'd enter the world of tubes...) and did some tuberolling to find the ones that would fit my setup and the Senn's the best - what can I say - sonic heaven !!!
Then... I got myself an AKG 701 about two weeks ago. The moment I connected it to the Darkvoice, I instantaneously wanted to send them back - they sounded as if a big capacitor was in the line. Airy, no body to the music, boring. Felt like you'd be miles away from the music and you'd hear it over the radio. Then I read in several forums about burning them in - 100, 200, 300 - even 500 hours. Well, did that with all kinds of music from Maroon 5 to Massive Attack to Miles Davis and Patricia Barber. Let them ran through night and day.
And really - each day, they got better and better. Never witnessed such a dramatic effect with any component I had before. They now have about ~ 100 hours on them and they begin to make fun.
Are they better than the Sennheiser's ? Well, it depends... How can I characterise the sound of them:
HD600: Close to the music, bass, deep bass, punch, details, lots of details, but not very spacy.
K701: Space, lots of space. Not too much bass but sufficiently, still some aggressive heights (Miles Davis hurts sometimes), punch primarily for drums, and very nice voices.
So my advice: Listen for yourself but make sure you listen to well-run-in cans. Both are worth the buy, but the AKG IMHO only if you've got a tube amp and are able to change the amp's sound characeristics through tube rolling (what I did).
So much for my three pence on the topic. I'm putting my 701's on again...