This is a frame compatibility question. Older frames will need a post mount brake while newer frames will have moved to flat mount. If you are looking at a new bike this isn't going to come up but it might for those retrofitting a new groupset. Keep in mind that there are adapters available.
If you have a post mount frame and are buying a new groupset it makes sense to get flat mount brakes. For one thing it's difficult to find post mount calipers at this point. Even if you solve that challenge though, flat mount is the modern standard. There are adapters to get a flat mount caliper to work on a post mount frame and if you upgrade the frame in the future it will be flat mount.