If I had this requirement I would just generate a file of specific size, place it on one server and on the other I would have a shell script running via cron and measure the time it took to download the file.
It seems like a relatively simple problem.
BTW are you sure you want to test download speed and not latency? I think some routers might have the later built in.
Definitely speed. My ISP runs on another service providers hardware and it bugs out from time to time and I get 1/10th of the speeds I usually have. My ISP has no way of knowing this so I have to know when it happens and place a ticket so they can place a ticket on the hardware guys.