Why is my pci-e wireless "N" card @ 150Mbps but when using an "N1" USB adaptor it runs at 270Mbps? - certified wireless usb

Both are certified Belkin 'N' and N1, respectively .. is loaded on computers, but not without a son, and tend to rely on
what is written on the packaging.
I have a Dell Inspiron 1525 with Vista installed .... wireless adapter I disabled the network traffic performance USB blistering but I was a little disappointed that want to PCI Express to 150 Mbit / s - what can I do? Change the channel number? or a unique configuration is (obviously) not in the user guide ... Regards