There's little difference between those, but usually what VCs are comparing are things that are more like 2% chance at $1b vs 2% chance at $200m -- and then there's a big difference.
Also, if one company is a 5% chance chance at $200m but 0% chance of being worth more than that, while another company is a 1% chance of $1b, the second company might still be a better investment because it's probably 1% chance of $1b AND a 5% chance of $200m if it doesn't go all the way to $1b.
Also, if one company is a 5% chance chance at $200m but 0% chance of being worth more than that, while another company is a 1% chance of $1b, the second company might still be a better investment because it's probably 1% chance of $1b AND a 5% chance of $200m if it doesn't go all the way to $1b.