If I had to chose one, I'd be shooting large format a all times. What attracts me beyond resolution and freedom of composition are the plethora of options to process film to bring out the best tonality or the most vibrant colors. Right now, there is no substitute.
There are practical considerations that make me soot medium format film and digital. For example, if I am in the studio and have to have something to show the next day, I will shoot digital. When I go places for the first time, I usually carry my film MF with a variety of different films. Main reason are compromise between mobility and ability to shoot reasonable photos. If I like something after developing the MF film, I usually head back out with my LF.
While there are some really great 35mm cameras out there that are well suited for candid and street photography, it is not my "thing". I have totally abandoned shooting 35mm three years ago after being spoiled by MF and especially LF.
To come back to the original question - I have mental check-list to consider before I pack my camera back. If I'm driving somewhere, I usually have my spare 4x5 field camera in my trunk ... just in case ;-).