I seriously don't get where the "ivao atc" sucks is from. I've seen plenty of twitch streams where the streamer flies on vatsim and I can't notice the diffrence between Ivao & vatsim, well except that everyone sounds like a robot on vatsim
But the whole "competiton" thing is just childish, By all means have a favourite network but do not harass the other network if you don't have any solid proof, or actually keep it to yourself and let everyone love their network in peace. One thing that you could take in to account is that Vatsim obviously "wins" if you want to fly in the US. USA is nearly always a wasteland on Ivao but there's the occasional atc online but I myself have never flown in the US on Ivao so I can't answer for the quality
My suggestion is : Try both. I originally started out with vatsim but the client was so messy so I switched to Ivao and haven't looked back ever since
