Replying to Headphones

So i was wondering if anybody here could help me.

I'm currently shopping for headphones and i am learning a lot about them. Like frequency, impedance, etc, etc...

My first question is: What is the major difference between a 60ish impedance and a 30ish impedance?
And second, what headphones would you recommend for a budget of 150 or less?

Please, do not say skullcandy. I'm actually looking at Sennheiser HD line, Shure SRH750DJ, maybe some Urbanears Platan, WeSC Oboe, and a pair of RCA HP350. That last one looks good, but don't know about it's performance...

Anyways, i appreciate the help.