Whilst there isn't much between them, from what I have seen I would persist with Johnstone in goal for now.
From the saves he has made, I find Bond seems to parry the ball back into dangerous places, were a couple in the first tie and I can recall one last night which was parried back into a danger zone.
Personally I think Johnstone looks more comfortable having the ball at his feet out the two aswell.
Competition is always good for a GK, something we haven't seem for a fair few years with Foster and Boaz.
But for now I would still be choosing Sam.