A service item by default posts to income.  And service items are at the top of the listing makes them easier to find using a drop down list on an invoice

a non inventory item by default posts to expense, you have to check mark it on the item screen to open it up so you can use it to post income.  Non inventory is great for things you buy and you sell since it can be used for both when you set it up that way.  

you can use either, just be consistent