Both Sony's are pretty well regarded for their price point. Personally I own the 7506's and I love them. They are also the highest recommended over-ear phone for under $150 by The Wirecutter - see here: http://thewirecutter.com/reviews/the-best-150-over-ear-headphones/
The V6's are similar, and some seem to prefer them over the 7506's. Reading the Head-Fi Winter Buyers Guide, you can see that they were the cans that got site founder Jude started down the path to being an audiophile, and as such hold a special place in his heart. See here: http://www.head-fi.org/a/2013-head-fi-winter-gift-guide-over-ear-headphones
Although note what he says:
"Is it the best at the price? No, and hasn't been in a long time. You can find headphones that isolate more, sound more refined, have better detail retrieval, etc."
I can't speak on the Sennheisers, but if it were me, I'd get the 7506's now as a solid, rugged, classic, low-priced beginner headphone, and then later on once you're hopelessly addicted and able to splurge more, get a pair of Senn HD 600's.