Whenever I go out for shooting seriously (as serious as I can get, that is) I have my Rollei and a Leica, since that is about all I can carry for any distance, with a tripod. They take different kinds of pictures, to state the obvious; either can make powerful portraits, for example, or landscape work, and entirely satisfactory prints in any size I care to print them (up to 16x20). I've been preparing for a road trip next spring to west Texas and had been wanting to take a 4x5, but the only one I have lives on my studio stand and is too darn heavy to schlep, I don't have a suitable tripod for its weight, either, and also Dennis's pictures of Mexico convinced me that the Rollei is Big Enough, so that's what I'm going with. I also have a Widelux which is fun to use and does some nice landscape work.
In ratio terms I'm mostly using the Rollei (I also have a Minolta Automat), next most the 35s, then the 4x5 (but it's getting winter and there will be a lot more of that soon). Just now looking at my gallery postings is a pretty fair representation of ratio.
Bill