I have used both grapples and thumbs, and in my opinion, if you have a grapple, you are wrecking, handling large/bulk material, or loading trucks. they work best on larger machines. and with an experienced enough operator, you can in fact do some fine picking. at the last company I worked for, buckets/thumbs were unheard of. we had some darn good operators who could pick fine wire with a grapple. the standard 3-2 tooth box grapple can't be beat for heavy demolition work, there are limited parts to break (I.E. Keepers, teeth, cylinders, hoses)
up until about 2 months ago, i had never used a hydraulic thumb, it does have it's place, however I don't care for the fact that it's hard to hold your scraper beam tight, because you have 2 cylinders one always wants to push the other one, (generally the bucket cyl pushes the thumb cyl.) although I love loading trucks using a hydraulic thumb over a grapple, because you can fold it up at any time and get it right out of the way. I also like the thumb/bucket combo for doing brick demo, because it's much easier to scoop brick than it is to pinch it.
I have also learned the value of a bucket's ability to do fine picking...though I still run my thumb like a stiff arm grapple most of the time: keep the thumb fixed, put it behind what you're picking and bring the bucket to meet the thumb.
finally I feel that a grapple has more of a use when picking from a concrete slab, but with a bucket you need to be able to actually get under the material a little in order to get a good grip.
so. both have their place, and a good operator can make efficient use of either one he is given. it really all depends on the particular situation. unfortunately, most contractors pick one and stick with it, I haven't seen too many that use both thumbs and grapples
Eric