Slattery knows all about the requirements of wearing the No 7 jersey for the Lions in South Africa having figured in all four Tests on the victorious 1974 tour.
And as much as the man who lost only once in a Lions jersey in 25 appearances wants fellow Irishman Wallace to prevail in Saturday's RBS 6 Nations championship decider at the Millennium Stadium, he knows that Welsh back row marvel Williams will be key to the Lions in two months time.
Speaking at an HSBC Lions Legends dinner in Dublin, Slattery said he was greatly looking forward to the battle of the top two No 7s in European rugby in Cardiff.
"David Wallace and Martyn Williams are different players. They both have different attributes," said Slattery.
"But they will both have roles to play with the Lions. Martyn plays a full 80 minutes, whereas David's weakness is that he sometimes doesn't.
"David is very powerful on the ball and very good on his feet, while Martyn is a great scavenger for the ball. The first thing any Lions side has do is get the ball.
"Speed is also important for playing in South Africa because of the hard grounds and stamina is another key factor. It is not a place to take slow and heavy players.
"You need a set of mobile forwards, not carthorses. That might mitigate against players like Adam Jones, John Hayes and Nick Easter.
"Half back is also going to be a very important area, although the good thing there is that Ian McGeechan will have some good choices to make."