USB device are tricky to select because their primary use is for mobile device which tend to have different priorities. On a desktop machine you want as much signal as you can get and size and power usage are secondary concerns.
If you were to dig in the fcc database you can get the output power but it is a lot of work. Generally the USB unit that you find that are physically larger and already have cables to extend it away from the case tend to be high output power also. Even if you find the thumbdrive size devices be sure to use a usb extension cable to extend it away from the case.
PCI cards in general put out maximum power and have larger antenna. Problem is those nice antenna are located next to the large metal box of your...