Hi Rhodes,
Both 3200 films are faster than 400 film by not much more than one stop. When I can live with 1600 shooting or even 3200 and contrasty, pushed images I prefer to use 400 film.
The same image, with 3200 films, gives me more grain (Ilford more than Kodak) and a bit more shadow detail.
Ilford Delta3200 is a bit faster than Kodak IMO (close to nothing), and it's also a bit lower in contrast, and can be well processed in DDX or Microphen. I don't like its look too much, even though I like visible grain in general.
Kodak TMax P3200 is the best fast film to me when it's processed in TMax developer: amazing tone, beautiful clean grain, and much higher acutance/sharpness than Ilford.
I'm getting with stand development (ISO400 film at 3200) higher acutance than with normal agitation development: one thing I wonder is, how well would stand development work for TMaxP3200 at 6400 incident on TMax developer... I'd bet it works... Shamefully I can't get TMax developer right now to try that...
Recently I've found only for night shooting those high ISOs make sense... On 99% of situations including low light, a fast lens and 400 film work very well... Even for night shots, with 400@3200 I can shoot at f/1.4 1/125, 1/60, 1/30... And the high contrast looks fine on night shots...
Cheers,
Juan