From a game point of view 2020 seems more realistic, from a business point of view 2019 seems like a better proposition imho.
If they release in 2019, MS cannot launch in that time. They'll have at least a year because MS cannot bring out a new console too early because of the XB1X. It also means Sony can Up-rez all their games to PS5 and not give MS's new studios time to be ready to promote their next console, because most of the new studios will only have been working on new games for about a year, if they launched in 2020, then Sony is giving them an extra year of free dev time.