I get why people like McClain. He brings an enforcer attitude to the middle and make some great plays. He is not better than a healthy Sean Lee though. Sean Lee does everything at an exceptionally high level. He can cover TE's and RB's, he is heady with tremendous instincts. He can shoot gaps, or beat blockers with speed or technique. He knows when to hold blockers and when to shed. McClain is a homing missile, but his all-around play isn't nearly what Lee's is.
I said it before the season and I will say it again, Sean Lee is the most important person to the improvement of this defense, even ahead of Gregory and Hardy. His versatility as an every down linebacker enables guys like Hitch, Wilber and McClain to a lesser extent, to fill niche roles. He also tandems with Church very, very well, as Lee will hold blockers and allow Church to shoot gaps.
P.S. I haven't gotten to the Wilber breakdown yet. This is my final semester and I am buried with Literature work. I'll try to find time after I work out.