Based on my experiences with both boards, and having both the President (D858 AM/SSB board) and the Uniden Washington (MB8719 single AM conversion board) along with the Cobra 2000 GTL (MB8719 dual AM conversion board), I prefer the D858 chassis for sensitivity, as both my President Washington and Teaberry Stalker II can RX a signal down to .32uV (.32 microvolts). Both the MB8719 boards drop RX at about 3uV (3 microvolts).
However, provided that you do not do the NB/ANL mod for the D858 chassis, the MB8719 boards do have better NB/ANL filtering.
For me, at least where I live adjacent channel rejection on the D858's and the MB8719's are very close, not enough of a difference for me to choose one over the other. I don't have any issues with my D858 boards being muffled in any mode.
It's really just a matter of preference. You go with what you like.
~Cheers~