I’m not an expert but my understanding is the true range the radio can area have is also dependent upon the height of the antenna and “line of sight” range. Hand held radios are limited to about 3-5 miles. Your masthead mounted antenna will increase that to 12-15 miles depending upon actual mast height.
Also, your hand held will have more limited functionality than a quality fixed mount VHF which today can offer AIS, DSC, and channel scanning/monitoring capabilities to name a few.
My recommendation is to have both, the fixed mount is your base in the boat, the handheld allows you to roam about and stay connected or to have one below and one in the cockpit. Admittedly my boat is much larger but, I have a standard horizon 2200 fixed mount with AIS, DSC, loud hailer, NMEA0183 for connection to my msd at the nav station, a remote mike for use in the cockpit, and 2 handhelds for use as needed including the dinghy and for communication when on shore.
Part of this is redundancy, you can never have enough radios, part of this is due to the nature of our sailing which may include being offshore, needing to hail large ships and even exploring remote places. Hand held have a definite purpose and can be very useful in a pinch, fixed mounts are much the same. They can replace each pother but also Offer complimentary capabilities and a level of redundancy which is always good.
Hope this helps and I’m sure I’ll be corrected on the true range of each as well as other things I’ve missed or (more likely) misunderstood.
Fair winds