My personal opinion is that the 91XG is better at dealing with multipath and reflections... Some people agree with me, others have had contradicting experience.
There's not a great deal of difference in gain between the two antennas but the 91XG is going to give you a bit more gain.
That you're seeing WCMH is a bit remarkable... your TV Fool report would suggest you would not. But as the bright minds that gave us TV Fool have already said "your mileage may vary".
If you're in the mood to try another antenna, the 91XG would one of the best for your situation. Rather that replacing the DB8, try both. Cable the 91XG separately, all the way to your TV. Then consider using an A/B switch or an auxiliary tuner.
http://forum.tvfool.com/showthread.php?t=2882 This would make for easier 'channel surfing'.
The 91XG mounted on a good rotator, and equipped with a good low noise preamp such as the Antennas Direct PA-18 would possibly produce other signals or at least periodic DX reception. If you do install a rotator, consider using a thrust bearing as well.
http://www.3starinc.com/tb-105_thrus...a_rotator.html