Get both. But to choose which one first... If you want to drive historic cars, or want the best overall driving & handling experience, and don't mind making your own events in Free Play or playing the online events & the fairly bare-bones career, then go with DR2.0. If you want to drive modern WRC cars and want the most immersive career experience with team management & car development, then go with WRC8.
I wont be able to get both, playing time is limited as are the funds.
The immersive career is swaying me towards wrc8, played the original dirt rally to death and dirt rally 2.0 looks basically the same again.