Both the FORS CoRe and FORS Relic have the same-size search coils with the following exceptions:
The smallest coil for the CoRe is the one I named an 'OOR' coil for its slightly Out-Of-Round shape of 4.[size=small]7[/size]X5.[size=small]2[/size] DD. The smallest Nokta coil for the Relic is the round shaped 5" DD as they describe, but I usually type it in as a 5¼" size as that's what it measures closest to w/o including the coil cover thickness.
Other than the shape of the smallest DD coils, the Relic does have one different search coil available, and that is the solid-body 5½X10 Concentric. Between the Concentric and Double-D solid coils of the same size and shape, my personal favorite is the Concentric coil. Not that the DD design doesn't work, just that the Concentric coil works 'differently' and provides me a certain edge in many applications.
As for comparisons between the two mid-size Double-D designs, the 5½X10 solid-body and the 5X9½ open-frame, I personally prefer the newer open-frame style. Why, you might ask? Because I find the looks to be more 'modern' or 'stylish' and I can feel a slightly different balance in its favor. Better still, the open design lets that coil be worked in a wading Beach Hunting application as it easily works smoothly in the water whereas the solid-body DD is more buoyant and doesn't have a sweep and coil presentation that is as comfortable as the newer open-frame style.
A positive point in favor of all of the available search coils is that they work quite well on what I consider to be the best 19 kHz model available on the market today ... the Nokta FORS Relic.
Just my experienced opinions,
Monte