Mid and low end are just terms that are thrown around; don't worry about it. Just know that the more you pay, the better gear you can get. That said, keep in mind that as you get to the more expensive stuff, you will start to see a lot less improvement and IMHO it's not worth getting a rig over $700. If you want to pay the premium for minor improvements, go right ahead, but I would at least test the waters before jumping in with a $1K+ rig.
The differences are trivial. The Q701 is in a different color, and some people claim it sounds slightly better. The K702 has a detachable cable. I would get whichever one you can get a better deal on, if you do decide to go with AKG.