Just remember that if you have a smaller car the tint will be more effective then on a bigger car. My mate who works at a tinting place has a comonhore and it has 20 percent on it and its not too dark, He put the same tint on a mitsubishi mirage and it was dark as anythin, both from the inside and outside.
remember this when making your decision. Also alot of good tinting places will have some glass that already has the tint on it so you can see what it looks like.
Cheers
Jason