My $Y$ is a count variable and I am estimating it using a negative binomial function. I am not sure how I can decide which model is better. In the first model I am selecting a value for $k$, and the second the MASS package is estimating a value of $k$ for me.

Will

anova(mod1, mod2, test="Chisq") 

work?

If so, which model is the null referring to?

Otherwise, is there a better way to decide which model is better?

library(MASS)
mod1<- glm(Y ~ X1 + X2 + X3, negative.binomial(1), myData)
mod2<- glm.nb(Y ~ X1 + X2 + X3, myData)