I have Sibelius 6 but have not played with it much. But the very first thing I tried, to see how Siblelius handles the horizontal placement of tuplet numbers, revealed that one is stuck with how Sibelius does it, whereas Finale allows more tweaking. (Here's that thread:

http://forum.makemusic.com/default.aspx?f=5&m=298254

). On the other hand, Sibelius's defaults are better than Finale's in some types of tuplets, and in fact Finale's placement could be described as "buggy" -- see the thread.

Probably most people are not going to tweak tuplets in this way, so Sibelius wins. On the other hand, for a "power user," as you describe yourself, perhaps this is indicative of less control in Sibelius than in Finale.

I don't think one can necessarily extrapolate from this one example, but it suggests the trade-off is that in Finale you can usually do exactly what you want--that's certainly been my experience--while Sibelius will give you something that looks reasonably good for less work (see "magnetic spacing") but with fewer options for customization. Sibelius's other strong point is that the user interface is easier to learn, but since you have already spent the time learning Finale, that shouldn't be much of an issue.