They cannot be compared directly, as they are used for entirely different skills.
When you hide a word, and show only the definition, you are testing your ability to recall.
I have a concept in mind; a smallish, round fruit, red in color, crispy when I bite into it, it grows on a tree, and Americans are said to make many pies from these fruits.
Can you recall the word to describe this word?
When you hide the definition, you are testing your comprehension.
What does the word apple signify?
As such, when studying, you ought to study both, as dictated by your language learning goals.
If your goal in language study is simply comprehension (perhaps you're studying literature in another language), then you may wish to focus on (possibly even exclusively) on the comprehension aspect.
If your goal is to have conversations, you'll really need both.
Note that these aren't the only two skills in vocabulary, either. For my flash cards (using Anki), I also have cards which present the spoken word, and to which I recall the definition, and cards which show me the definition, and I have to produce the spoken word. For some words, I also have a card where I must spell the word, after hearing it spoken. These are all different aspects of vocabulary, and the different memory skills tend to strengthen each other, by building stronger memories. But once again, which of these skills you want to learn may depend a lot on your language learning goals.