The difference between both the Dirt series and WRC series are:
1)WRC to me is a bit more enjoyable but a lot less realistic than Dirt.
2)The handling of the cars in Dirt tend to mimic their real-life counterparts better than WRC.
3) Some of the courses in WRC - especially WRC6 are better looking than some of the courses in Dirt in my opinion.
It all comes down to preference really. I do consider Dirt to be the better rally title overall but WRC is a very solid and enjoyable alternative.