What is the difference between the virtual network cards you offer? Which do you recommend?

By default, your VM will use the Intel PRO/1000 emulated NIC, as it is the most modern and is widely supported in modern operating systems. However, wider support for the Realtek RTL8139 card means that we recommend using the Realtek emulation if you experience any problems with the Intel model: Windows XP/2003 users in particular may find that the older card is better supported on their OS.