From all I have read and based my personal experience, Rodinal is not the best developer for push processing. Tri-X and Rodinal give a very specific look. Salgado used this combination for a long time and many of his prints, while gorgeous, look like they were photographed on sandpaper. Contrast can also build real quickly.
My normal film is Tri-X, but last year I shot a small amount of HP5 for a project. The first thing that struck me about HP5 was that it appeared to have lower contrast than Tri-X, which should work in your favor, if you're push processing. I'm seeing the same in your two shots.
I don't think you're going to get an accurate assessment from this test, because of Rodinal. It's a great developer, but I don't think it's the best choice for push processing. Tri-X @ 1250 in Diafine is pretty impressive. Smooth tonality, good shadow detail and surprisingly fine grain. Obviously there are the usual suspects, Xtol, DDX, Tmax developer or good old D76.
If you're looking for a 400asa film to push I highly suggest TMY-2 400. It has a very linear curve and the grain stays very small. Great stuff.