The advantages of under the bump are that they aren't so bulky under tops and to combat exposure or to allow you to wear a shorter top you can always wear a belly band or long vest top under your top top. Under the bump tend to work better for skinny, stretchy style jeans as the stretch in the denim will hold the jeans on your leg as supposed to having to rely on the waist band only.
If you've ever been pregnant, you know that maternity jeans can be a real saviour for your wardrobe. As your waistline expands, everything starts to not fit quite so well (sob!), and it can get uncomfortable—not to mention expensive. But if you have some decent denim options at your disposal, you can at least wear some of your looser tops over your bump. And there are some brilliant jeans that can help with that.