Ive done a 1 peice and it wasnt hard at all. you just have to lift the front of the car higher then with a two peice.
then you push the header up from the bottom and hang it on his bolts.
i did it alone and it took me about 1 hour for everything.
also 1 peice is better because there are less areas that could cause turbulance on them. I.E where the two peices come together could not be a perfect match thus slowing air flow.
Personaly i would never buy a two peice.