Should I use http or ftp links in apt sources.list

You're not going to save much time using one over the other, but if efficiency is what you care about, use HTTP. An FTP client has to send a username and password and receive acknowledgments for those, and then issue several commands to set up the file transfer and receive acks for those, too. All this back and forth magnifies the effect of network latency; with 100-200ms ping times to the server you could waste a second setting up the transfer. With HTTP, the client opens the connection, sends one command and the file transfer starts immediately.