I would take a hydraulic FEL grapple over a hydraulic bh thumb.
We're certainly thinking alike in that I also love the hydraulic grapple. It is lighter than the FEL bucket thus increasing the lift capacity of the FEL.
However, I don't see where one is a better option than the other.
When digging with the BH, the thumb makes quick work of grabbing the rock or stump and lifting it from the hole. It is also very nice for wall building given its narrow width. You can get the BH bucket and thumb into tighter spaces when placing a specific rock on the wall. A grapple is typically much wider than a BH bucket.
Having the grapple and not having a thumb during a BH digging effort would require me to turn the tractor completely around to then use the grapple: which may not fit well enough into the tighter hole that was just dug to grab the unburied item.
The exact reverse comes into play when manipulating a pile of brush. The grapple's additional width makes it so much faster and it gets a much better hold on the brush for transporting purposes: very few drops. The BH bucket and thumb are fine with single tree limbs, but they take much smaller grabs when attempting to grab from a large brush pile: and they tend to suffer more drops in those instances.
I see both being unique to their task, both are superb implements, and both are well worth having. If I had to choose, then yes, I would go with your selection and opt for the grapple. It is more versatile especially if one becomes fairly skilled using their BH bucket as a set-up unit for the grapple. However, I would not want to choose. I'd want both.
And, while I am listing my desired implements, I should include a hydraulic post hole digger: preferably one that ran off the backhoe hydraulics. PHD's are great for planting bushes and trees, and, of course, for fence post holes. You get the perfect hole, the exact depth desired, vertical walls, and, when placed on the BH, you get more reach, two holes from one parking spot by simply swinging the boom side to side, and a better line of sight vs looking over the hood and under the FEL arms.
The Gardener