Re: RDS-A vs RDS-B
Buy whichever shares are cheaper since they have the identical economic claim on the company. Both classes of shares are subject to a 15% withholding tax on the dividend if the distribution is taken in cash; however, shareholders of either class of shares can elect to receive the dividend distribution in shares of RDS-A (but not RDS-B).
Hence, if you make your initial purchase in RDS-B and elect to take dividends in shares of RDS-A (to save the 15% withholding tax), you will end up with positions in both classes of shares, which is a slight nuisance.