Both titles feature a free copy of older games in the series, while Steam's Tropico 4 offer also includes some additional bonus content.
Stronghold 3's pre-order offer on both Steam and Direct2Drive includes a free copy of the original Stronghold to play immediately, as well as an exclusive level each. Steam's offer includes the "Tower of London" stage, while Direct2Drive users gain exclusive access to the "Edinburgh Castle" stage.
For Tropico 4, a free copy of Tropico Reloaded is available immediately with the order from either Steam or Direct2Drive. Steam, however, features a few additional bonuses: the Isla Nublar island and El Presidente "Luchador" avatar costume.
Tropico 4 is scheduled to hit the PC and Xbox 360 in August from developer Haemimont Games. Stronghold 3 lands on PC from Firefly Studios in September.
Disclosure: Direct2Drive is owned by Shacknews parent company GameFly Inc.