I think toss will be a big factor tomorrow, the fact that England will like to chase in this high-tension match as their batting is batter and Pakistan will like to bowl as their bowling is better, this will make things even more interesting. And pitch will be a big thing as well, we saw how track was slightly turning in Adelaide, will have to see how MCG wicket behaves, if it's good for pace then England's chance will drastically go down as Paki's pacers will get a lot of help.
Of course the toss will play an important role. But since there is a chance of rain in the match and if it is known even before the toss, I think whoever wins the toss will prefer to bowl first. Because if the game starts with cut over due to rain, then those who bat later will get a bit more advantage. The toss will greatly effect the outcome of this match.
Edit : England won the toss and chose to field first. As expected.
England's batting line-up is good, but we need to know the same couldn't be happening all the time. If a break in partnership is possible at the beginning, then surely the batting will slow down. Just think of the match against Ireland. In tomorrow's match toss will have much importance, however this is gonna be a win for Pakistan according to me.
It's not just that England lost to Ireland, Pakistan also lost to Zimbabwe. England faced a lot of trouble in that match due to rain. But if this match can be a full over game then I hope to see a competitive match.