They could really do with a head start, but I don't think they'll get one. Sony are well aware that launching second could hurt their chances at capturing a new generation, so they'll be looking to launch soon too. PS4 will start it's decline and they'll want to fire back at the Switch with a new console. Microsoft can't release too soon after the X so I think they'll be both forced into late 2020 releases. If Microsoft think they can ride the X1X for a year against the PS5 then they are taking a huge gamble and likewise if Sony think they can ride the PS4's popularity against the X2 then they will be risking losing a generation and really having to fight to get it back.