My first DSLR was a Nikon D70s - fantastic camera. I had a few photos printed pretty large from that camera, including one at 4' x 3' (1.2m x 0.9m), and it still looks great today. The D70s has a 6.1MP sensor. The reason the photos from it still look better than any phone today is partly because of the physical size of the sensor, which allows in a TON of light and the grid is spaced so that each pixel is rendered clearly (unlike any phone on the market), and partly because the lenses are quite a bit better.
If you were to look up close at an image printed from a physically larger 4MP cellphone sensor like the One and a smaller 8.7 like the L920 side-by-side (assuming the lenses and processing software are of about equal quality) when printed large and only paid attention to the objective quality difference between the two, I would be willing to bet that they'll either look the same, or the One might be a touch better. Step back a bunch, and they'd probably look about the same, but I'm sure I would prefer the photo from the L920 - I like the character (subjective qualities) that the Zeiss lens/whatever sensor they sourced gives the photos that phone produces. Put either of these up to even a ten-year-old DSLR, and that will blow them away. I got a 920 because I love the photos it takes, but I still keep my large camera and lenses for when I want real quality work.
If you're at the point where you're thinking about printing large and (especially) selling your photos, a dedicated camera is probably the way to go
Even if you were to pick up a cheap second-hand DSLR and a couple of cheap primes like a 24, a 35 and a 50, you'd really start to see the difference when viewing photos up close and printing them. You'd also have the option of trying out different focal lengths so you're not stuck at 23mm all the time if you decide to get into portraits or telephoto work.