Generally, most bars are 3-pce. This would mean that at the strut tower, you'll have a piece which will fit over to fasten the nuts to, and the cross bar will bolt to these end pieces. Well, you loose alot of rigidity by this as there is a pivoting point as apposed to a one piece design which will be extremely rigid, yet very difficult to install due to the tension now you'll create from fastening one side down, and trying to align and fasten the opposite end.
For functionality, a one piece design is best, but may not look as nice, and is more difficult to install.
For a little of both, you'll see more 3-pce. designs available, and they look nice.
The only problem is if you ever get into an accident, being hit from the side along the front...knock on wood....that impact will follow through to the other side, affecting both shock towers. This will be more expensive to repair as the shop will need to straighten both sides now.