I'm a total whore for large batteries. The added capacity and ability to deliver lots of current will give you a nice rate of fire, decrease trigger lag, and ensures you have plenty of juice to play long, extended sessions without having to worry about recharging. I also tend to lean towards longer barrels because of the added range and accuracy they provide. And I love the solid feel of a full stock.
But which gun you get also depends on your size. If you're shorter, a full size rifle with a fixed stock may be awkward for you to carry, shoulder and manoeuvre properly. I'm 6 feet tall, so a large rifle is no issue for me. Also, consider if you'll be playing CQB. A full size rifle is very awkward in tight spaces. For that, a carbine with a sliding stock is better.