I've never heard the senns, but based on comments on Head-fi they are the best closed can you can find in terms of blocking out noise. I do own a pair of 7506's and they block out an average amount of noise...i wouldn't use them on the subway or bus however. They are great for tracking because their quite analytical due to their bright nature. They also offer a good amount of bass, once you familiarize yourself with the sound you can trust them to point out any defects in a recording you made. They also fold up and come with a pleather pouch to stuff them in. By the way, the MDR-V6 is the same exact headphone, sound the sames, looks the same (minus the gold plated plug) and sells for $30 cheaper. If you do go with the Sony's make sure to contact Beyerdynamic and order a pair of replacement VELOUR ear pads for their DT250's. They fit on the sony's and boy are they comfy unlike the pleather ear pads on the sony's
Run a search on head-fi (in the headphones forum) for comments on both cans, you'll find plenty!