If investing in an IRA, buy the "B".
Otherwise you get a credit for foreign taxes withheld on the "A" shares, not in an IRA or tax advantaged account.
Also if you reinvest the dividends they will buy "A" shares with your dividends and then if in an IRA you will be out the tax money as there is no way to claim for foreign taxes withheld inside an IRA.
Originally ther were to separate companies, Royal Dutch and Shell transport and Trading which held all of the shares. They have substituted the "A" & "B" designations under the Royal Dutch bannner to replace those former entities.