Tower enroute control is similar to the preferred route in that both are designed to improve the system effeciency. FAA defines the Tower En Route Control (TEC) (in AIM section 4-1-19) as:
TEC is an ATC program to provide a service to aircraft proceeding to and from metropolitan areas. It links designated Approach Control Areas by a network of identified routes made up of the existing airway structure of the National Airspace System.
Preferred routes, on the other hand (from Instrument Procedures Handbook) are defined as:
Preferred IFR routes are established between busier airports to increase system efficiency and capacity. They normally extend through one or more ARTCC areas and are designed to achieve balanced traffic flows among high density terminals.
This outlines one of the major differences between the two: in TEC route, you never 'talk' to the ARTCC. As flyingmag notes:
TEC flights are intended to be relatively short — generally two hours or less, and are conducted below 10,000 feet. Throughout the route, communications are managed through tower controllers, and departure and approach controllers at a TRACON facility. You will never talk to ARTCC.
Another important thing is that the TEC is essentially equivalent of getting a pop-up IFR clearance, as already noted (from AIM section 4-1-19):
There are no unique requirements upon pilots to use the TEC program. Normal flight plan filing procedures will ensure proper flight plan processing. Pilots should include the acronym “TEC” in the remarks section of the flight plan when requesting tower en route control.
The TEC is established only in areas with Radar coverage. US DoT WP AT 7110.5A specifically notes:
TEC should be established/expanded only within areas of radar coverage. In areas without radar coverage ... an equivalent level of safety is maintained, TEC may be established and/or expanded.
TECs basically are published preferred IFR routes that are entirely in TRACON airspace. As a result, most of the TECs are in the northeast and the west coast (which can be found here), where the airspaces are close enough that one can fly with Radar coverage the whole way.