which fiio to purchase or alternative

I'm looking to replace a small cheap headphone amp ( runs on 2x AAA batts ) I purchased from Maplin a few years ago to run in the car and on a pair of headphones 

used 
On an apple Touch and a Nano
On Car Stereo via the Aux input - current amp works fine
in the car I also have a USB charger 

Sennheiser PXC-450 Noise reduction headphones 
Bass boast maybe useful - when on noise reduction loses a bit of bass

I have been looking at the Fiio E range and yep, a little confused 

I would like to use while charging - I know some do - But if theres a big advantage for the modek that does not charge than happy to put up with that

I like the extended battery life 80hrs mentioned on one of the models 

I will also purchase the ipod cable, as this is mentioned in a few reviews as improving quality 

so what do i know 
I'm tempted by the 
E7 - £50 
Batt life 80 hrs , 
no noise hiss, 
can NOT be used while charging 
150mW 16ohm 

E6 - much cheaper £20 - 
still no noise hiss, 
can be used while charging 
battery life 10hrs - 
poor build quality compared to F5 - but F5 has hiss 

E11 Approx £40
can NOT be used while charging 
battery life 10hrs 
battery that is easily replaceable
300mW - 16ohm - so is this really twice the power of the E7

E10
now very confused 

so a little confused on what will work best 

Do not want for PC sound - just the headphones 

any help will help - thanks
also any alternative suggestions

well, i decided on the fiio E7 and my daughter got it for me for xmas along with the dock and ipod cable - so just charging it up and will be playing later today / tomorrow 
i have a 1.5 hr trip on my own tomorrow (taken motherinlaw home ) so will give it a good test in the car - which was going to be its main use