I use a 98 Custom Platinum with a Flatline barrel and really like it. I used to have one on my original 98 Custom too and even though it was bulkier at the time that one worked fine as well. The only thing is they are picky on paint, I recommend Marballizer or other thicker shelled paint as the Flatline is harder on paint than most barrels due to the ramp. Also if you go with the Flatline get a nice soft type of squeegee, if you use a cheap plastic one you will scratch up your barrel and ruin it.
My brother used to have a Freak Jr. kit on his 98 Custom, before he switched to his Vertical Stock Class Phantom, that was the most accurate barrel and marker I had seen on our field. It is much less picky on paint since you can choose the insert appropriate and under bore every time. Since it is a straight barrel you won't have to worry about breaks as much and when they happen they won't be as problematic for you.
In the end, they are both great for different reasons. The Flatline allows you to shoot further in a straight trajectory, which can be very nice for when you have to shoot through a small clearing in the trees or brush and cannot arc your shots. While the Freak will give you a more flexible system that will allow for the best paint to barrel match every time. Essentially, if you want to reach out and touch someone, Flatline. If you want a more consistent barrel, Freak. Hope this helps!