I've owned both and can offer some useful advice.
The J-mo is my go to ski as it's an acceptable park ski (a larger skier could get by with it as their primary and be perfectly happy), and it's very fun all mountain in variable conditions. It's stiffer and wider than the FR, is not symmetrical, and is much much more versatile.
The FR is a very fun park ski. Light weight, nice flex, symmetrical and durable. Center mount it and shred the park, done.
If you are looking for a park ski, get the FR. If you want a ski that you can ride in literally every condition and have fun, the Jeronimo is hard to beat. Great in the park and groomers, but can be skiied fairly hard in crud, hardpack, and powder. Back when I had more skis and more days a season, I rode the J-mo on days when it snowed and bit and I wanted to ski all mountain in the morning and lap the park in the afternoon, and the FR most of my days on the mountain as Colorado has endless sunny park days. I don't ski park as seriously anymore and the J-mo is now my everyday ski.
The Jeronimo can best be described as the perfect compromise between a true park and a true all mountain ski. Thus, you get a very fun ski that will do well across the board, but can obviously be bested by a pure park or pure charger/powder specific ski if you push it very hard in either category. It's like it was made for Summit County.
