This is really up to your personal preference. Gold electrodes offer better conductivity but do not last as long as Silver electrodes do because the gold tends to wear off. Silver electrodes last longer but do tend to oxidize over time reducing their effectiveness. Silver -silver-chloride electrodes only have a cosmetic disadvantage in that they look like black burnt metal.

Regarding flat disc vs cup electrodes. The cups are an older style, and were designed to hold the EEG paste, but the newer flat style has proven much easier to clean and has no trouble holding enough paste to work as effectively.

Electrodes do take a beating over time and should be replaced every few months or when showing signs of wear.
Be sure to clean your electrodes gently after each session. Also see cleaning electrodes.