The 6416 is just an HD DVR. The 7216 is a HD DVR and can have the Multiroom capable. If you order the Multiroom DVR you will get the 7216. The Multiroom feature is just a software switch that is either turned on or off with the 7216. As to the Handshake issue, the 72xx series seems to have more issues than the 64xx series. I had the 7216 when I first signed up 2 years ago but promtly got a new box (6416) because the issues were to bad to watch tv. Since getting the 6416 I get the often reported DU1(which is actually DVI) when turning the box on and minor volume fluctuations. Still waiting to get the 1.7IMG to see if it fixes the volume fluctuations, but if it doesnt, it wont bother me because they minor enough that I can live with it.
Either the 6416 or 7216 can be a multi-room (this is just a software switch). The 6416 is an older model (no longer manufactured since it violates the FCC requirement for "seperable security" - cablecard) that has less memory and no RF output. As was stated the 7216 seems to have more issues with the HDMI handshake problem. My 6416 never had a problem but was switched out for a 7216 (unrelated problem and the change didn't fix the issue - long story) and immediately had issues. The 7216 also seems to render SD with more sharpness than the 6416, but that difference has been minimized since the last 2 IMG updates (1.6 and 1.7). With the update of the IMG to 1.7 the 7216 gives a stretched 16:9 guide, info bar, etc. while the 6416 still shows it in 4:3 aspect ratio (no difference in the guide data or number of hours shown in one screen, just the way it is presented).
There are rumors that the 7216 may (and I stress rumors and may) with some future update support ESATA for an external drive, but again that is only a rumor.
From a functionality and recording capacity they are, for all practical purposes. the same.
Either the 6416 or 7216 can be a multi-room (this is just a software switch).
I didn't know the 6416 had the Mutliroom functionality. I was told by a Tech that the 7216 was the Multiroom box. Good to know.
There isn't a lot of difference between the QIP 6416 and the 7216 as far as the customer is concerned, at least at this time.
The 7216 has a slightly faster processor (essentially a Quad Core MIPS engine), and more memory (128mb versus 32mb), I believe it also has native support for MPEG 4, which the 6416 lacks.
As others have reported the 7216 seems to have more hand shake issues with HDMI and perhaps a slightly better SD picture.
The Ethernet port, USB port, and AV input ports are all disabled by Verizon on both devices, so you are limited to the built in 160gb drive.
It has been variously reported that there will be a new DVR STB in the near future from Cisco/Scientific Atlanta. The Cisco/Scientific Atlanta units have a lot more CPU horsepower under the hood, so hopefully a number of the existing issues will be easier to deal with.
The HDD size is going to have to go up in the near term, because 160gb hdd's are going out of production at most manufacturers in the immediate future. I doubt that will mean anything for existing 7216/6416 users.