I have been to both over the last 4 years (twice to each). Have yet to be successful at either unit. Both are steep and deep. Trail travel is generally by ridge lines.
I hunted 17 @ Nez Perce Pass on the Magruder Corridor road. It was a bit thicker with timber and underbrush but I saw elk taken and heard plenty of bugling during the early (9/15-9/30) season. Easier access than 27.
I hunted 27 @ Thunder Mt. A little less underbrush, a little bit drier a little more open terrain. Access is much more remote and early snow is a real possibility. Almost got caught in the backcountry one year. Saw elk taken but in talking to the game folks in McCall the elk are on the comeback and not necessarily populous. This is my preferred hunt of the two just for the remoteness and lack of hunters.