Look at it this way....
If you are planning on getting only one bucket, get a grapple bucket. You will regret it later when you order one in a few years, you will pay more because, well, the cost of everything goes up always. You will pay exhorbitantly more in shipping a single item VS getting it shipped with your initial tractor purchase, and that alone will offset much of the cost of purchasing it in the first place.
If you are planning on getting a minihoe, a thumb on the minihoe is almost a grapple, but unless you pay someone for aftermarket hydraulics, its not a grapple, its a fixed position thumb. While useful, it won't carry the amount of material that a grapple will carry. And sometimes it will be in the way, requiring you to pin it out of the way if its moveable, or removing it alltogether if its not a moveable thumb.
A grapple bucket with teeth can be used for grappling, hauling loose material, excavating harder material, snow removal (keep the teeth up), firewood hauling, etc.... all with one attachment. I seriously wish I would have purchased one. My attempts to make my small bucket into a grapple show that the bucket is not reinforced enough and I would have to add serious bracing to counter the effects of the hydraulics. While I can do it, I just wish I would have done it from the start.