The proportional brake controllers used to be a very high priced option, now the prices are very near the same as for the time delay brake controllers.
Nice article on the differences between the two. Trailer Brake Controller Information | etrailer.com
Trailer brakes are electric tow vehicle brakes are hydraulic. The time delay controller simply provides an ability to set a delay and adjust how hard the trailer brakes are applied. Needed because electric trailer brake action is instant, hydraulic TV brakes take time to compress brake fluid and increase braking pressure. Without delay you would get yanked back by the trailer brakes coming on first and harder.
Proportional use technology to detect rate of tow vehicle deceleration from braking and feed trailer brakes the correct amount of electric current to provide proportional amount of trailer braking.
One thing to consider is where you will mount a proportional braking unit in the tow vehicle. Because proportional has to detect deceleration front to back the less expensive units have to be mounted closer to level and in a straight line front to back. More expensive units can be mounted so that they are angled up or down and adjust to not being level. At a more expensive price point they can function at more extreme angles.
Half way down these pages the angles are diagramed which may make this clearer.
Brake Controller for 2005 Escape by Ford | etrailer.com
More expensive model.
Brake Controller for 2005 Ford Escape - Tekonsha 90885
One other feature of note in the more expensive model, it cuts down power to 25% if stopped for more than 5 seconds, such as at a traffic light
. Helps keep the trailer brake activators from getting hot in stop and go driving.
I'm not able to say one way or another if etrailer is good site to purchase from but they have good documentation.