Slightly different point of view here, but I have both and 95% of my playing time is with DIRT.
Reason why? - really you do get a very good sense of speed while running through very narrow fdirst roads, the damage physics are excellent as are the variety of cars (be it old MK2 escorts to newer ones).
Remember these are two very different styles of racing and two very different sports. I do feel connected with both games in terms of racing, but DIRT for me is a lot more rough and ready I feel.
The tracks differ a lot also which does add something rather than just tarmac.
Ok, the multiplayer is not great but it does only go to recreate what a rally would be IE not very often are you racing at exactly the same time.
They are both 2 very good games in my eyes