My choice is both.
they both have their own pros and cons.
NFS MW:
pros
the open word structure is amazing.
auto log is my favourite in that game.
lots of card available right from the beginning.
stunning gameplay.
crash physics is good.
the open world has been created with perfection
cons
handling is not very attractive.
crashing feels too much at some times and too less at sometimes.
the damages doesn't reflect in handling
too easy a game
Grid 2
pros:
very hard (finishing first) when you reach season 5.
handling is superb.
the graphics is amazing
cons:
crash physics is not up to standard.
takes a long time to complete.
not much vehicle choice(not much from my favourites)