Well, I own a pair of audio technica sj55 and I've tried the mdr v55. The SJ55's will block out more sound than the v55's and I think they leak less as well. Both have punchy bass, but I would say the Sony has more of it. Defiantly the basshead set of the two. The Audio Technica SJ55 are also really well built, with no creaking, unlike the V55's that creak like my old ST-550's. But then, Sony MDR-V55 are quite a bit more comfortable, to me at least. They both take some getting used to, but the SJ55 have so much clamping force that you don't even notice that there is no cushion on the head band. Also, the sound of the Sony's in general have slightly less detail than SJ55's. This is gonna be a tough choice my friend...
Here's a little break down.
1. Audio Technica SJ55
Pros: Detailed, warm sound. Isolate noise well. Sturdy build quality. I've heard some prefer them to the M50's, though they shouldn't be compared ($70 vs $130)
Cons: vocals is overpowering for some (not me though). Tight clamping force. WES may occur (Warm Ear Syndrome).
2. Sony MDR-V55's
Pros: They look sick. Like seriously, are you a DJ? Classic Sony soundstage with moar BASS, though the mids and highs are still very nice sounding (though slightly veiled when compared to the SJ55's). Decent build quality (not as study as the SJ55 in feel however). Red and black.
Cons: What ear pads? (They feel straight off of the ZX100's). Creak, creak, creak.
Remember, they both aren't especially comfortable and both need some getting used to, though I find the sj55's less comfortable. I ended up with the SJ55's simply because of the sound signature. Good luck!
Update: I now own both pairs, and it's really close, but I would go for the sj55's. Just as I said above.