In the UK at least, Deliveroo is almost everywhere, and Uber Eats is more or less only in cities where Uber is a licensed minicab - which is most major metro areas, but not smaller towns and so on.
From that alone, I'd guess Deliveroo is by far the bigger player in this market, but Uber Eats possibly has a wider international market?
I think it's fair to say Just Eat is the major contender to Deliveroo over here. Based on the absolutely tragic UX alone, I doubt many people bother with Uber Eats
I'm so glad it is not just me. There are 2-3 restaurants that are only on UberEats. Every time I open it, I hate it. The Uber ride app was/is the pinnacle of good design; UberEats has extremely poor information architecture that makes using it painful.