A typical bumper sticker is 11" x 3". Generally you want the person in the car behind, while driving or waiting at the traffic lights to read what you want to say.
The simple way to do that is use a simple big text message.
That may be OK for "Save The Whales", or "I love any shit Apple makes!" but is that all you really want to show and say?
The images here shows 2 of 8 options we were working with recently. If you just want to show the URL and a message of what that business does, then the top one may be fine. It will be easy to read from a distance and get a message across, but would it get someone to actually visit the site and look at the style of graphics samples?
Now the bottom image is more complicated, not so easy to read, but also has some visual samples and the URL. Would it get a second look? Would it possibly get someone to actually remember and look up the URL when they get home or to the office?
We have used the very affordable zazzle to make this bumper sticker to try out.
We don't have hard facts to support that one is better than the other, other than a bigger sign than a bumper sticker is better for business advertising on a car!