The Sennheisers have a warmer sound without quite as much detail in the higher frequencies compared to the AKGs. The AKGs also seem to have a bit better soundstage and stereo imaging, at least to my ears. The Sennheisers are known for having a very smooth, flattering presentation of vocals.
I just did a little comparison using my tablet, which doesn't have a very powerful headphone amp, and actually both of these headphones were pretty similar in terms of the volume I get. Both of them just barely reached a decent listening level with the tablet's amp all the way up, so either one may be a stretch to drive with your practice amp.
If you can only have one set of headphones for music and your guitar amp, you might consider the Sennheiser HD58X or HD560. If you have your heart set on a nice entry-audiophile set of phones, get the HD6XX or K702 and order a $30 set of Koss Portapro's for your amp and phone. I think You'll be amazed how well the Portapro's compare.