(Only one person's opinion, hopefully others will reply too.)
I don't use an entrance reducer, though at times I know I should be. An entrance reducer helps the hive by: 1) giving it a smaller space to have to guard, 2) keeps mice out during the winter. I'm not sure why the two different sizes. I'm thinking the larger for most of the year, and the smaller for winter.
What I did instead, since I made my own hives, bottom boards, and lids, is I made the bottom board reversable. One side makes the opening 3/4 inch, and if I flip it over it's 1/2 inch. So far I haven't had mice get in the hive, so maybe I've been lucky.