Audio Technica ATH-A500 vs. AKG K450

Hello Gurus

I'm looking for a pair of closed backed, over-ear cans up to £150ish. I'm mainly gonna be using them on my laptop, occasionally on an Ipod Classic, and eventually on an Android 4.0 tablet perhaps by Samsung or Motorla when/if they come out.

I've narrowed it down to these two and like the look of the AT cans the best however the AKG's have a 32 ohms impednece vs 64 ohms in the AT's.

Is this significant considering my chosen sources? Will there be a noticeable difference? My Shure 420 buds only have an impedence of 22 ohms and I'd like it if these cans could produce the same volume and fidelity.

I'm not averse to using a pocket headphone amp like the FiiO series but would prefer to avoid it if possible.

Thank you in advance for any help

Seb