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.

Assuming that those numbers are known precisely, the 5%/$200M is clearly the better option. But in practice it's basically impossible to distinguish a 1% success rate business from a 5% success rate business. On the other hand, it's pretty doable to estimate best-case outcomes based on the size of the market being addressed and some assumptions about margins and market share derived from the business model.