I like variety (a "right tool for the right job" kind of thing), but if I had to stick to one, I vote concert. My justification is admittedly a little funny: it's not so much about what I prefer but rather what seems right.
Guitar is my primary instrument, so my fingers are happy with a larger instrument. Additional frets make an instrument more versatile to me. And if I A/B ukuleles just on sound, bigger always sounds more guitar-like, fuller, and "better" to my ear. Better resonance and sustain make picking sound better to me. So why concert? Well, I don't want my uke to feel or sound like a guitar. I already have guitars for that. I want my uke to be compact, like I'm playing a toy but taking it very seriously as an instrument. And I want it to sound bright and percussive, what I imagine as characteristically "uke-y" (for the same reason, the strings I use are not the strings that I think sound best). All these reasons point to soprano, but a cutaway concert represents (for me) a nice compromise: still small enough to be cute and islandy, but with a little more sustain and versatility than a soprano.
Hopefully this is not digressing from topic too much, but I really like how in threads like this there is a diversity of opinions without snobbery. That's a real strength of the UU community!