Either developer will work well; the choice is mostly about the look you want.

For Delta 400, D-76 is often the more natural starting point. Delta is a tabular-grain film, and D-76’s solvent action tends to give smoother negatives with finer-looking grain. If you chose Delta 400 for its cleaner grain, D-76 is a sensible first choice.

Rodinal is also perfectly usable, but it typically gives a sharper, more acutant look with more visible grain. Many photographers like that character, especially when they want a crisp, gritty rendering.

So the practical answer is:

• choose D-76 if you want smoother tonality and less apparent grain
• choose Rodinal if you want more bite, sharpness, and visible grain

Dilution also affects the result: higher dilutions generally tend to lower contrast somewhat, which can help with very harsh or contrasty lighting. There isn’t one universally “best” dilution here—use the normal recommended dilution/time for your chosen developer as a baseline, then adjust based on the look you prefer.