Alfa USB adaptor or pigtail and external antenna

Hello,
whilst I may not have many posts, I have been reading about for a while and registered yesterday.
I own a white AAO 150, which I picked up a couple of weeks ago from John Lewis with 2 years guarentee for £170

Anyways where I want to use the netbook alot, I struggle to get a good signal, at the moment I'm connected and it says I dont have any bars, thats how weak it is.

I have been trying to decide whether or not to get the Alfa USB 500mw wifi adaptor with a good antenna, or put a pigtail onto the existing pci-e card and have an external antenna.

Obviously the Alfa would be better for not having to open the netbook up, but what I am wondering if the internal wifi card has got a high power output.

Thanks