Both are rodents and the difference is due to species difference. However, why not try to test both species and see if there is a difference. I would suggest to use dogs which is more similar to human in case of laxative.

Most research uses rats in laxative experiments. But there are a lot of experiments using mice. Some use both

I prefer mice in my experiments (hepatoprotective) because they are smaller in size; less costs and large numbers can be used in a small place; reduce the use of plant extracts and do not tend to bite.

But, It is difficult to treat(oral administration) and The amount of blood is less

Rats are by far more intelligent and well developed than mice. Working with rats will definitely give more trusted results. However, for toxicity studies only, I may personally recommend mice, but generally, I may say that rats are better. I personally work with rats more than mice. Good luck.