I don't have storage problems on my iPad Air, but on my android phone, I use a couple of apps to help identify folders and apps that are disk space hogs.
On your iPad, if you haven't done this already, start by going to Settings -> General -> Usage -> Storage
to get an idea of where your disk space is going. This information will make your decision 1000x easier, as there are likely things that are eating a lot more disk space than you expected. Similarly, you won't have to agonize over deleting a favorite app if it only requires 20 mb of space.