This is a common question -- which should I buy for my Garmin GPS, City Select (CS) or City Navigator (CN)? First of all, go to Garmin's website and check to be sure that your unit is compatible and capable of auto-routing. Otherwise you'll be paying for features that you can't utilize.
Here's the lowdown on the differences. CN was designed specifically for certain StreetPilot units; those units can take advantage of some sophisticated voice-prompted routing information in CN. Though CN can be ran on many handhelds, the map segments are much larger, with several exceeding the 24 MB limit of a number of units. This is not just a problem in those areas; it makes it difficult to construct a trip along a linear route (can you say road trip?). To further complicate matters, Garmin is discontinuing CS, which has smaller map segments, replacing it with CN. This has caused quite a stir, and Garmin handheld owners aren't happy about it. Apparently the company is listening though, and they seem determined to shrink the map segments in future versions of CN.
CS is getting harder to find as a result of this phase-out. If you can only find CS Version 6, don't worry. As long as it is purchased (and unlocked) after June 1, 2005, you are entitled to a free upgrade. And there is some good news in all this; Garmin has cut upgrade pricing for CN from $150 to $75 and on CS from $75 to $50. My advice? If you've got a handheld, buy CS.