Personally, I feel like Arnhem is a less serious contender than Maastricht though.
Maastricht and Rotterdam seem most eager to host it, Maastricht started promoting ESC in the city even before Duncan's victory and back in 2014 they said they'd like to host if The Common Linnets would win as well. They are REALLY serious to get the gig. A couple of weeks ago I visited the city and there is a huge 'EUROVISION SONG CONTEST 2020' sign at the MECC venue, the tourism board of Maastricht often post about ESC, and they released an limited edition t-shirt for promoting the contest as well. They even got André Rieu to promote the city as host city.
I also think that the Arnhem region is less appealing, outside the Randstad area. Maastricht is that as well, but is heavily internationally orientated with big German and Belgian cities nearby. And, Maastricht is more known thanks to the Maastricht Treaty.
In all honesty I can't really see ESC being hosted in any other city than Amsterdam, Rotterdam or Maastricht. Maybe Utrecht will surprise but at the moment I don't think so. I read that the province of Utrecht isn't really eager to pay for it and that there is a chance they won't even hand in their bidbook.