Omnitik is a decent outdoor (weather proof) AP with mediocre antenna gain:
antenna patterns document implies that it's antenna gain is 0dBi or very similar. It's vertical pattern is slightly directional (so it does matter how stations are placed around AP vertically) but not so much.
mANTBox 15s, on the other hand, features a very high-gain antenna (as name implies, it's up to 15dBi),
product brochure includes antenna gain patterns which support this. Also beware that mANTBox has quite narrow beam in vertical direction which means you might have to mechanically tilt the antenna to get stations inside main antenna lobe.
Which means: mANTBox will have quite longer range at expense of covering only 1/3 of a full circle. Business case: coverage of "sparsely populated" area (such as camping place / caravan park) with 3 devices, each covering 1/3 of a full circle (together they very well cover full circle).
Whether this resembles your use case it's up to you to decide. If you need the long range that mANTBox provides, then add another mANTBox with some distance between them, operate it on very different frequency and orient both so that you'll cover the 180° you need. Just beware that in the overlapping area you won't be able to control stations about which AP they'll connect (unless you configure APs with different SSID).