Hi GS, I'm 5'3" & 120 lbs & have test paddled both boats you are considering - very recently, in July, at the Great Lakes Sea Kayak Symposium.
The Mystic will be a *much* better fit for you. It is a true small scale seakayak that can take any big water paddling you desire as well as calm lagoons.
The Fathom LV has a *much* higher deck which is a detriment for you in two ways a) your knuckles might rap on it b)high decks catch the wind so you will be paddling harder.
Also, Eddyline designs are North American style, meaning they are larger and roomier than British style, like the Mystic. You'll not have the contact you want and need to get the most performance out of your boat. Yes, you can pad out w. minicell but once it's more than an inch of padding, what's the point? Also the cockpit may be too deep and there's no remedy for that except to raise the seatpad which compromises one of the things you have going for you as a petite female - a low center of gravity.
Lastly, the LV is "small" kayak - for an average sized man! You and I are much smaller than that. We don't need a beamy high decked boat to horse around. Because we are lightweight a boat that is oversized for us will mean we are floating w. the waterline too high (too much showing beneath the seam). That makes the boat overly sensitive to wind and waves. Again - more work, less control.
Both boats are fine boats, built w. impeccable quality. But fit is everything. Think small and sleek and if it is between these two, the Mystic is the clear choice. That is a silly salesman's statement that the Mystic is a "rock garden boat"... that means it is playful and agile! you can do everything a paddler can in a longer seakayak except pack unsupported for a week's trip, and how often will you be doing that?
Other boats you can consider: Avocet LV, Current Designs Willow,
P&H Capella 161, Hurricane Tampico 140S. Maybe a Romany LV. If you can demo the Tiderace Excite S.
But if you like the Mystic and the way it handles, it's an excellent choice for you!