The hobbyking batteries are the best, and thanks to @Sander and @Stey @Eviltwin-NL who brought me to it.
11.1V gives you high rate of fire but the question is : does your gearbox can handle this burst?
The other thing about 11.1V gives your better trigger response and that's why most CQB players prefer 11.1V, but the down side about it:
1. Will out run your BB's and empty your Mags faster then the 7.4 battery.
2. large and can't fit crane stock or buffer tube and for that you will need crane stock pouch.
So at the end it's worthy? if you ask me i will say No, and advise you to use 7.4 because it's more healthy to your gearbox and save your BB's and can fit easy most crane stock or buffer tube (condition the dimensions 116x17x13mm).
Withe all of that the 7.4 charge faster than the 11.1 and discharge lower than 11.1, therefore all together 7.4 is the best choose for you.
If you still think the 11.1 is your favorite, i have FireFox 11.1V 2400mAh Li-Po Li-Polymer Military Battery 20C almost new i can give it to you in good price.
Size - x2 = 125mm x 20mm x 15mm
But still I am telling you the 7.4v is better than 11.1 for beginner.