text classification : comparing classification reports
Okay so keeping it very short and precisely in context of your question- 

Accuracy tells us, out of all the documents how many are classified correctly.

Precision tells us out of all documents which are predicted in a category, how often its correct.

Uni -gram- "nasa", "is" "space" , "agency"
bi-gram- "nasa is", "space agency"

Now lets go over the numbers, in both the cases accuracy and precision doesn't have significant difference.

But as  we can see bi-grams can give me much more information and hence can have better performance on unseen data. Try to test the model on unseen data/validation set and compare the difference.May be Try tri-grams etc also.