In general, both Tomtom and Garmin offer great coverage, and their maps will cover most, if not all popular areas. However, both companies use different map providers, resulting in some slight differences. Tomtom purchased Tele Atlas back in 2007, and now exclusively use Tele Atlas maps, while Garmin uses the Nokia-owned Navteq as their map provider. Many claim that Navteq has slightly better coverage in the US, whereas Tele Atlas has better European coverage, but you should check to make sure your location is covered well by either provider.
One problem common to both map providers is that sometimes one-way streets are improperly marked, and as a result the navigation GPS unit may try to lead you down the wrong way. Tomtom's Mapshare feature allows users to report and share map errors and updates with each other, which can be useful in instances such as this.