I mainly use 35mm, but have always wanted medium format (the Holga just wasn't doing it for me) and 4x5. Photos are my memories, and I find 35mm perfect for that purpose, especially on vacations. Most of my photography is snapshots.

Now that I have a good medium format camera (Bronica GS-1), it's place for me is portraits, and for photos that are very deliberate (or to enlarge for display purposes).
Either format can deliver what I want, but when I'm running around taking random pictures, the 35mm fits better. Medium format is for when I have more time. Although the quality of such a large negative is clearly an advantage, it's not large enough to supplant my 35mm for most of what I do.

The sad thing is, when I finally get a Super Graphic, there is a risk that most of my snapshots will be on 4x5 - I'm funny that way.