"Also, I'm wondering if the front and rear sights are removable, in case I want to change the sights for more accuracy."
ditto what everybody said, really no difference in accuracy potential
the shorter 5.5" barrel balances well in hand, and carries better (if you carry in holster walking woods or whatever)
I prefer the longer 6 7/8 barrel for the longer sight radius, helps the shooter make the most out of their own abilities when shooting open sights, better methinks if this be a range pistol
tapered or bull barrel, grip style... all just a matter of individual preferences
The Mark III comes with a weaver rail which can be left permanently mounted (leastways the 6 7/8" model did), so can quickly/readily mount or dismount scope or red dot of choice without need of removing the open sights, and can leave weaver rail in place when shooting open sights...
but you can quickly remove rear & front sights also if that happens to be your preference, just less convenient if/when switching back and forth
I like a Millet 1" tube 3MOA red dot on my MKIII (maintains a low profile), but sometimes do pull it off to shoot open... it does not balance well for me if/when offhand shooting w/ the red dot mounted, balances just fine without
(my 10" bull barrel MKII is simply not an "offhand" shooter, too much barrel weight, so I keep a 2X pistol scope on that one, awesome accurate)
PS
the Buckmarks are very nice too, I just happen to favor MKs