comparison seems to be such that it cannot work as the only tag on a question and this is one of the criteria for meta tags. Therefore, the proper question might not be if it should be a synonym to another tag but whether it is justified as a tag at all.
Looking at some questions tagged with comparison they seems to do just as well without the tag. Thus, the tag does not seem to add relevant information which defeats the purpose of any tag.
There are cases where it does add relevant information but these cases seems to be cases where the questions are more appropriately tagged file-comparison.
Here is an example to further clarify. If I want to compare technology x and y (both for which there are tags) it is sufficient to tag the question x and y. There is no reason to also tag it with comparison because x and y being alternatives already implies a comparison.