BFG:R is better, but the rules are unofficial so you need your opponent's agreement to use them. Traditionally, the Eldar ships move in the move phase, shoot in the shoot phase, and then move again in the ordinance phase. In MMS, they move twice in the move phase and none at all in the ordinance phase. So they lose some hit-and-run capability, but the MMS sheild rules make them more durable.