You want to know whether a word is acceptable in formal conversation, only slang or antiquated. Timing is everything. Look for the most modern words and see which dictionary has them and whether the definition is current. Language is a living, breathing thing, and you want your dictionary to be as lively as possible. Dictionaries are generally updated every year and overhauled every 10.
Some publishers go for quantity, others for depth of definition.
Figure out how much word count matters to you and how the books stack up against one another. Bells and whistles. They include punctuation and capitalization guides, sections on biographical and geographical names and medical abbreviations. Look for what you think will help you. Where you will keep your dictionary and how you are going to use it should be a big factor in what you buy.