Audio/Visual & Electronics - radio din adapter

I have a '99 civic and I got a new alpine single din radio for my car, and there are 2 different adapters I could use to put it in and replace the old factory radio. There is the one that came free with the radio from crutchfield with the slot beneath it that has rounded ends, or there is the one with the rectangular slot beneath the radio.

I was wondering if one adapter is better than the other or if it's just a preference thing between the rounded slot or the rectangular slot?