35mm cheaper? Not really... I have more invested in my rarely used 35mm system than I do for my medium format or large format equipment. Canon "L" lenses cost about the same or more than lenses for medium or large format cameras. And used MF or LF cameras can be picked up quite cheaply.
Reasons for my choices on what format to use:
Large format 4"x5" for landscapes, scenics, or architecturals when the light is "just right," there's ample setup time, and hopeful the images captured will be wanted for large prints. Always used on a tripod.
Medium format for the exact same usages as above, but with less setup time available, not as perfect light conditions, or multiple films are wanted to be used for the scene (by changing backs... usually carry 3 different backs with different films), and ease of use. Always used on a tripod.
35mm film used for action, or street, or photography "on the run" where tripods can't be easily used, or the use of the motor drive is essential. Hand held or used on a monopod.
Digital Point & Shoot for generic shots, grab shots, or for pretesting settings prior to burning sheet film.