Both are good bullets & both will work, however for Elk I would pick the Accubond for sure.
For me, I would look at terminal performance more than BC for 2 reasons in this case.
1. An Elk is a large animal & alot of penetration may be needed and sometimes at a raking angle. You used the Sierra on Hogs & it worked, but the length pf penetration can be more in the Elk. At 308 vel. the Sierra usually does OK, but I would rather have a more consistant bullet. I used that bullet at 30-06 velocities & took sev. Whitetails & had 2 jacket/core separations.
2. BC is important sometimes, but secondary in this case because at long range the 308 energy gets pretty low for Elk after 300 yds. & up to 300 yds. BC will not matter much unless you Sierra tip gets flattened. This is not to say that the 308 won't take a rib shot Elk over 300 yds, but I don't consider it a long range Elk gun either. However with a lead tipped bullet as the Sierra, a battered tip will change that BC alot, no problem at 100 yds, but is at 300. So the BC part is a yes & a no, but not as important as terminal perf.
IF your rifle will shoot the Accubond, I would save your Sierra ammo for local hunting. I have done alot of Western hunts & I know the bullet is the least expensive part of the hunt, so talking about the cost diff. would be silly. Why not stack the odds in your favor as much as possible?
I do agree with Lloyd that the Gameking at 308 vel. is a great Deer bullet.