Good to see you giving it a try, By the way, I'm modeling the new Z4 coupe that just came so it's a little different to the tutorial that you're following.

Anyway, to answer your questions. You use both types of curves for different situations. Bottom line, whichever curve you use, try to build it with as few CV's as possible, and as cleanly as possible (i.e. the curve should be flowing with no severe, sudden shifts in curvature). When you use the birail or squar tool, Maya is going to try to follow your curves as closely as possible, which is a good and bad thing. It's good for accuracy, but bad because if you built your curves badly, it'll still build your surface for you, but badly also (like in your picture). Maya doesn't know what your intentions are, so it just follows the guides you give it. You can rectify your problem by either building the curves again, but more cleanly, or by rebuilding the surface using the "rebuild surface" tool. Problem with rebuilding the surface rather than the curve is that you'll lose the construction history from the birail, or square.

There's so much with nurbs that can trip you up, you just have to go through and experiment. One suggestion I'd give is to put the car model aside for now, and work on something a little simpler to get used to the tools. Go through EVERY nurbs and curve editing tool that Maya has, and really get to know what everything does. That way you'll know the best tool for the job to solve a specific modeling problem. There's aspects of the tutorial that you're following that I think could have been done better/cleaner, so it's really all up to how you model, and the skills that you have in your toolbox.

mmoore, it usually takes a couple of months for me to finish an entire car. Mostly due to the fact that I work my regular job about 10 hours a day. If I were going to pick a car to start with for nurbs modeling, I'd suggest the new VW beetles as they are constructed with nice large flowing surfaces that lend themselves to nurbs modeling.