be a toss-up

be a toss-up

COMMON People say that something is a toss-up when they think that two things are equally likely to happen or be chosen. She might go to Scapa. Or Rosyth. It's a toss-up really. They said it's a toss-up whether oil prices will go up or down over the days ahead. Note: When you toss a coin, there is an equal chance that the coin will land heads or tails.

be a ˈtoss-up (between A and B)

(informal, especially British English) be a situation in which either of two choices, results, etc. is equally possible: ‘Have you decided on the colour yet?’ ‘It’s a toss-up between the blue and the green.’
This expression refers to tossing a coin in order to make a decision about something.