To keep the tests as equal as possible, I used the same simcard and antenna for both modems. I used a terminal program to send the AT commands specified in the manuals of the modems. With both modems I downloaded a file from the speedtest.tele2.net FTP server to the internal storage, and measured the time this took. I repeated this test 3 times with both modems, getting similar results at every try.
I couldn't find anything in my method that could explain the big difference, so I was hoping anyone on the forums could help me out.