DiRT 4 Chief Games Designer Hits Reddit Today For an AMA

What Game is worth getting? Dirt 4 or Dirt Rally..
That's impossible to tell because Dirt Rally has been out for a year now whereas Dirt 4 has only recently been announced.

Dirt Rally is a small game, but it's a very well-made game. If nothing else, you should be able to pick it up cheaply and get some longevity out of it, at least until Dirt 4 is released in June.
What Game is worth getting? Dirt 4 or Dirt Rally..

If you have a wheel and any system that can run Dirt Rally and want a challenge threading the needle best you can to not destroy your car and ruin your run, Dirt rally is an awesome game. It looks great, runs well but can be punishing.. this also makes it very satisfying if you enjoy Rally and are willing to work/struggle to improve.

If you want an arcade feel with the graphics and would rather race trucks, buggies around the track as well as doing rally, then Dirt 4 would be better getting. If you enjoyed Dirt 2/3 this will be much the same but better with also the ability to turn on the hardcore sim physics of Dirt Rally if you so choose.

Also look into SLRE if you have a console, it is really cheap on Amazon right now. I got it for $13 and it is arriving today.