Your present wiring supplies the combined phone/ADSL signal to all your sockets. With your present wiring, it doesn't matter which is the master socket, all behave the same way. (see more below).
Since the signal goes to all the sockets, you can plug your ADSL modem into any socket. The microfilter just acts as an adapter providing a suitable RJ-11 socket for the connection lead.
The second socket is a filtered BT socket for the phone.
Can you plug a phone/Sky/fax/whatever straight into a phone socket without a filter? It depends on the equipment, some are designed to ignore the ADSL signal or just work anyway due to luck, others suffer interference. My house alarm, for example, is hard wired to a socket but works fine. if you'd prefer not to use a filter use trial and error and see if it works.
The make and design of filter is immaterial. Some people prefer the rigid plug in variety, as with Solwise below, others prefer the
BT box on a bit of string where they can hide them behind the furniture.
Other Options If you intend to plug your modem into the BT master socket, you have a further option. You can replace the existing BT faceplate with a
Solwise ADSL splitter faceplate. That will provide an ADSL socket on the faceplate, but will filter the signal to all other sockets in the house, removing the need for any plug in filters at all.