It's a tricky question to answer because each one has a different dimension to it. Yet, after giving your question serious thought and consideration, I would say that keeping kosher takes precedence because it is a Torah obligation, whereas reciting Kaddish for a parent is a Rabbinical edict. However, having said that, our Rabbis through the ages discuss the incredible benefits that the soul of a parent receives when Kaddish is recited for them — that is not something that should be discounted easily.
I’m not sure from your question what the conflict is between keeping kosher and saying Kaddish. If at all possible, barring any unusual circumstances, I think that the “real answer” to your question is that they should not be looked at as an “either/or” situation, but rather they should both be done!
I hope this reply is helpful, and please feel free to write again regarding this or any other question about Judaism.