The 74TXVi seems to be a replacement for the 54TX, and not the 56TXi. There's no detachable cord, no double-layer copper chassis, and the 56's gold plated jacks are gone as well.
In terms of connection options, the Pioneer absolutely trounces the Marantz. 1394, HDMI, XM, and iPOD, the Marantz has none of that. Also, the 74 has the latest gen MCACC, which is WAAAY better than Marantz's "MRAC", which seems to be an after thought.
The Marantz definitely outpowers the Pioneer though. If the Pioneer's somewhat limited amplifiers dont bother you, I think its the better way to go. DVI switching is basically useless at this point.