First consideration: Many states now have laws that outlaw scanners in vehicles (but most exempt ham radio operators). Do your research!
Next: Different scanners have different capabilities, so what you purchase depends on what particular systems you want to hear. There are still plenty of 'plain-jane' radio systems around the country, but most big cities (and more and more counties/townships) are moving to more sophisticated radio systems - which require a more sophisticated (read expensive) scanner.
Some scanner models are capable of interface with a GPS receiver. Properly done, this would allow the scanner to seamlessly switch from system to system as you travel along your route. If you're planning a long trip, this might be something to consider.
Best of luck,
- John