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.
Ok so I HATE maternity clothes... or I should say hated... I bought my first pair of maternity jeans yesterday from primark of all places and I am literally in love! They are still nice and skinny and fitted but so comfy around my bump - check out my maternity video on what I purchased here; https://www.youtube.com/watch?v=r3bLjGSHMZE&index=3&list=UU-DxGKz7B0xh_zSDzgnL2cw