I'm all Ai, all my work is done with Illustrator. That said, it's not easier, just a different way to use somewhat similar tools, that may be easier for some folks - like me
. I don't like working with layers and I find Illustrator matches my brain's way of seeing things. Just look at Don's work to see the difference. He has many complete illustrations both created in Illustrator and PS, but as you look at them, you'll begin to notice the difference. PS can allow you to much more effectively accomplish realism than Ai, but because of the tools in Ai, you get that "vector" look which is what some artists are looking for, or they need to scale the project up to a very large size. Vector art can be scaled without quality loss, while allowing the artist to work with a relatively small file when compared to a similar file in PS.
Some others might want to chime in on this part, but both programs have a pen tool and I think that for cars, it is extremely valuable to most artists who use Adobe products - PS or Ai. It may play less of a role with other subject matter but because of the hard lines often found on cars, you'll want the pen tool in your arsenal. For a second reason, if you dig the outline look, toon-style or not, the pen tool plays another important role. You will draw your outline and use stroke path. Ask away on any of these points and people can break it down the way they do it. For Illustrator, the pen tool is the only tool, without it, Illustrator is not the direction you want to go. I'm exaggerating, but it's a truthful enough rule of thumb. I'm going to post a tutorial in the new tutorial section and I'd say, the more of them you can read the better. They are great for new ideas on how to attack your drawings. deviantart.com is chuck full of them, and I would also point you to this link:
http://www.motorburg.com/forum/forumdisplay.php?f=32 Look at the tutorials from lemorris. I promise you'll come away understanding the pen tool and how it applies to drawing cars in PS and Ai to some degree.