Phrase to describe high contrast in the possible outcomes
I don't know if it fits your situation, but all-in is a gambling term which can be used to describe the situation
http://idioms.thefreedictionary.com/all+in
Other gambling terms can also be used like double-or-nothing
On a more general note, high stakes can also be used (commonly heard in both Gambling and Investment Banking)
http://www.macmillandictionary.com/dictionary/british/high-stakes