Question : Comparing cable broadband and adsl 2+ performance using two routers

I am currently connected to the internet via an ADSL modem router.

I am about to migrate to broadband delivered via cable for a much
faster downstream connection (although slightly slower upstream), and will
be receiving a cable modem and a new router.  I will have both ADSL
and cable broadband connections active for a short period so that
I can test the new connection, and
I would like to be able to compare performance by switching
between routers and doing some simple testing.  Two questions:
1)  Is there a simple piece of hardware that will allow me to alternate
between two ethernet connections without having to physically
unplug the cable each time?
2) Other than speed tests and pinging different IP addresses,
what tests should I run to compare the two connections, ie can
I test latency, etc.?

Answer : Comparing cable broadband and adsl 2+ performance using two routers

1) yes, its built into every operating system that I know of.  

On windows you can deactivate a network connection by visiting the control panel > networks > Network Connections and right click on the connect to disable / enable it.

In linux I am sure there is also a nice window to do it in, although I only know how to do it via command line.

ifconfig eth0 down
ifconfig eth0 up

2) It will be a lot of work getting a more accurate connection speed than just using a speed test server already available.  Most speed tests do a ping test before they start to show latency.  My favorite is speedtest.net.  I dont think its worth the trouble or the thought to do much more than that.  If you use the same server on speedtest.net you should get a relatively accurate view of how each connection reacts.