I think it will be difficult to compete with alternativeto.net which has a longstanding community, and options like filtering based on operating system. What are you doing which differentiates you?
Having filters to better filter through the large list of alternatives is on my TODO list.
But in general, I think both (and many other directories) can co-exist without issues. I personally think alternativeto is a little mainstream anyway, whereas my directory is focussed on people who explicitly want to selfhost, but obviously there is overlap.