Is there a way  to compare networks in terms of internet connection? I want to compare two different networks and choose the "better one". Better could be that I want to find out if the one I want to switch to would handle VoIP applications, like a Skype call in particular.

Is there any way for me to compare two networks like this? I have two SIM cards and and an Android phone with CyanogenMod 7.2
You can always perform a speed test and check for packet loss( Speedtest.net, pingtest)

Alternatively, you can try calling the skype Voice Testing Service which should be in your contacts from before, and if it isnt there the username is echo123 i belive.