In a perfect unit, I think filling a cow tag is easier than a bull tag. There should be more of them and hopefully they're grouped up. Them being together makes it easier to spot them and for there to be more shot opportunities. Obviously, you have to deal with more eyes and noses as you stalk in.
I think it's also important to note the "Hope" factor. For me personally, if you're in a unit without a ton of animals, it can be easier to hike around looking for a group of cows day after day compared to looking for a legal bull or buck.