I also don't understand novabolt's comment about DC's problem regarding technical performance (both are locked/near-locked 30 fps).
But it also is quite disingenuous to ask if FH3 would run as well with a massive weather upgrade a la DC. It would be like asking if DC would run as well (with the same visual fidelity) with an upgrade making it open-world. It really is a non-argument.
I've played and very much enjoyed both games, and in my eyes they both hit high notes regarding graphics in different areas. FH3 for image quality and skyboxes. DC for weather and overall details. I think both games have great car models and great lighting in their own ways.
Overall, I'd say FH3 is more "consistant" in the graphics department, whereas DC's high are higher and DC's low are lower. IMO, of course.
Edit: I guess I'd give the edge to Driveclub, because when everything comes together, the game is literally jaw-dropping.