Studies that could be unveiled next week are likely to zoom in on one or the other.
Oddsmakers favour Sagittarius A*, the black hole at the centre of our own elliptical galaxy that first caught the eye of astronomers.
Sag A* has four million times the mass of our sun, which means that the black hole is generates is about 44 million kilometres across.
That may sound like a big target, but for the telescope array on Earth some 26,000 light-years (or 245 trillion kilometres) away, it's like trying to photograph a golf ball on the Moon.
The other candidate is a monster black hole -- 1,500 times more massive even than Sag A* -- in an elliptical galaxy known as M87.
It's also a lot farther from Earth, but distance and size balance out, making it roughly as easy (or difficult) to pinpoint.
One reason this dark horse might be the one revealed next week is light smog within the Milky Way.
"We are sitting in the plain of our galaxy -- you have to look through all the stars and dust to get to the centre," said McNamara.
The data collected by the far-flung telescope array still had to be collected and collated.
"The imaging algorithms we developed fill the gaps of data we are missing in order to reconstruct a picture of a black hole," the team said on their website.
Astrophysicists not involved in the project, including McNamara, are eagerly -- perhaps anxiously -- waiting to see if the findings challenge Einstein's theory of general relativity, which has never been tested on this scale.
Breakthrough observations in 2015 that earned the scientists involved a Nobel Prize used gravitational wave detectors to track two black holes smashing together.
As they merged, ripples in the curvatures of time-space creating a unique, and detectable, signature.
"Einstein's theory of general relativity says that this is exactly what should happen," said McNamara.
But those were tiny black holes -- only 60 times more massive than the Sun -- compared to either of the ones under the gaze of the EHT.
"Maybe the ones that are millions of times more massive are different -- we just don't know yet."...