Currently, I am shooting Ilford Delta 400 at box speed. And I only have D76 and Rodinal developer at hand.
Which developer should I use better? And at what dilution mix?
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This will be a personal artistic decision - it is not like one of the developers would work and the other would not. Both are fine, and both are different.
Having said that: the typical use case for Rodinal are classical grain films (in Ilford lineup this would mean FP4/HP5+). It is a high acutance developer, producing unmistakably grainy (not unpleasingly so, but none the less visibly grainy) pictures.
Tabular grain films (such as Delta 400 you mention) usually benefit from a dash of solvent in developer. This gives appearance of a finer grain and smoothens the rough edges a bit. D76/ID11 contains sulphite, functioning as a solvent.
As a summary: there is no "wrong" option, but since you are paying extra for tabular grain my suggestion would be to try D76 first, and Rodinal second.
Neither developer is better: they are different.
Rodinal will tend to produce rather sharp, relatively grainy negs, while D-76 will give you significantly less grain at the cost of perhaps less sharpness. D-76 negs will tend to look rather 'smoother'. In both cases you will tend to get less contrasty negs at higher dilutions (so, for instance, if you took pictures in very hard light like bright sun, you might want to use higher dilutions).
As I said, neither is better, they are different. Which you choose depends on what you want your pictures to look like, and this is simply a choice that you need to make as a photographer.