They are both illegal, a straight pipe is usually refering to a pipe in place of the muffler and a test pipe is a pipe in place of the cat.
I have a testpipe from
www.testpipe.com they are good quailty here is a pic of what I got.
However I don't really like it. The power change is not really enough to have to put up with the shitty sound it produces IMO. I only put this on during the spring/summer when I am planning on going to the track. But that is just my opinon.