Search the message board as this has been discussed several times.
Briefly,dividend returns are higher for RDSA (same dividend, lower RDSA price). RDSB dividends have no foreign tax on them. RDSA dividends do have a 15% tax...but that is recoverable as a tax credit when file Fed. taxes if shares are in a regular account (non-retirement account).
RDSA is sold on the NYSE, the other is not. There seems to be a big price difference but that is partly due to dollars vs. euros. The dividend ends up the same after converting all to dollars. IF you live in US, a 15% foreign tax is always deducted from your dividends before you even receive them. When you file your income tax return you can easily take credit for any foreign taxes paid, so in a way it is a wash.