last year when we picked Joe at seven but he was probably our best player," said Johnson.
"There is not a weakness in Lewis' game but Joe's defence is good and his ball-carrying is particularly strong."
Johnson's other change to the side from the Ireland game was the expected inclusion of Leicester lock Louis Deacon in place of the injured Simon Shaw.
But there was a surprise on the bench, with Leicester's uncapped halfback Ben Youngs preferred to Paul Hodgson.
Youngs, 20, went head-to-head with Hodgson in Leicester's 35-19 victory over London Irish on Saturday and, like Armitage, he was also surprised to be selected.
"It is a harsh call on Paul Hodgson who has been fantastic for us on and off the field. Ben is playing well enough and we feel he deserves a chance," said Johnson.
"I told Ben he was on the bench and he said 'but Leicester haven't got a game this weekend'.
"I had to say 'no, you are on the bench for us!'"
Murrayfield has been a graveyard for England in recent years. Ben Youngs' father, Nick, won six caps for England at nine and lost there in 1984.
Saturday's game will mark the 20th anniversary of Scotland's 13-7 Grand Slam-clinching victory in 1990 and England has been beaten on each of its last two visits to Scotland.
Adding extra spice to the occasion, Johnson goes up against the former England coach Andy Robinson, who is now in charge of a Scotland side smarting from three straight defeats.
"They will be very confrontational but they are not afraid to try things so you have to be very alert," said Johnson.
"Their driving game has improved since he has been there. It will be a typical England-Scotland encounter in many ways.