Is the performance of the internal wireless improved if you install the correct driver?
sudo apt-get install firmware-b43-installer
sudo apt-get purge bcmwl-kernel-source
Remove the USB, reboot and let us hear your report.
The connection speed reported by
iwconfig and therefor by Network Manager, is notoriously imprecise. My card currently reports 1 Mbps but will happily download at the maximum available from my ISP. Please test with speedtest.net.
Checking your paste results, I see no obvious reason that either device won't achieve higher speeds. I have three suggestions that may be helpful.
First, I suggest you blacklist the driver for the internal in order to test the USB. From a terminal:
echo "blacklist b43" >> /etc/modprobe.d/blacklist.conf
echo "blacklist ssb" >> /etc/modprobe.d/blacklist.conf
modprobe -r b43
modprobe -r ssb
Second, I recommend that your regulatory domain be set explicitly. Check yours:
sudo iw reg get
If you get 00, that is a one-size-maybe-fits-all setting. Find yours here: http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2 Then set it temporarily:
sudo iw reg set IS
Of course, substitute your country code if not Iceland. Set it permanently:
gksudo gedit /etc/default/crda
Use nano or kate or leafpad if you don't have the text editor gedit.
Change the last line to read:
Proofread carefully, save and close the text editor.
Finally, check the settings in the router. WPA2-AES is preferred; not any WPA and WPA2 mixed mode and certainly not TKIP. Second, if your router is capable of N speeds, you may have better connectivity with a channel width of 20 MHz in the 2.4 GHz band instead of automatic 20/40 MHz, although it is likely to affect N speeds. I also have better luck with a fixed channel, either 1, 6 or 11, rather than automatic channel selection. Also, be certain the router is not set to use N speeds only; auto B, G and N is preferred. After making these changes, reboot the router.