Between those 2 I would take Jackson. I feel he is a complete physical beast at WR that has the potential to be a top tier dominant reciever. I wont take anything away from Holmes, hes great- but Jackson has the slight edge on him IMO.
Still, its not an easy pick since there is such great players on the board. I really think Greenway would be a great fit with us, but there is somthing to say about getting the first WR off the board half way down the first round...