Great idea doing a side by side. My initial comparison of the two (Over HDMI):
HD PQ: tie (though my memory is fallable here)
SD PQ: 6412
Heat: Tahoe is cooler
Sound level: 6412 quieter when cool, Tahoe quiter when hot (6412 fan is annoying)
Remote response (using same remote on both): Tahoe by a mile
FF: Tahoe faster and smoother, quicker response
Resolution setup: 6412
Recording time: Tahoe, 160GB vs. 120GB
External drive: Tahoe (eSATA)
Input config: Tahoe, component pass-through, audio passthrough
OSD: 6412, more status info
Let me know what you come up with, on your side by side. For me, the benefits of having larger hard drive and eSATA, plus good remote response and no lockups (I'm lucky I didn't throw the remote through the screen or crush it on the 64121) outweigh the downside of poor SD PQ and inferior resolution/letter-pillar box options. If I never have to look at an SD commercial again, I'll never miss it.
Maybe some firmware/software updates will improve the SD issues. Glad to have another Bresnan user on thread!