There is absolutely no difference between a P65 and IQ160 when shooting tethered other than a tethered IQ charging the battery.
almost every time i shoot on location- i have a computer to tether to, and the laptop screen blows away a camera screen.
There are actually many differences. Whether they are worth paying the premium for is of course up to the user.
When shooting with a P65+ tethered you can see the most recent tethered capture on the small screen and have few options for reviewing that image.
With the IQ you can see the most recent 10 captures. You can zoom in to any of those 10 images up to 400% and accomplish that zoom by tapping on the part of the screen that you want to see magnified (rather than first zooming to the center and then panning around as with most cameras).
My model/portrait shots are nothing special, but when I'm doing them I notice that the connection to the model (especially my less-experienced ones) is very important and any break in the action can break the rhythm and connection. With a P65+, in practice I would always do any image review on the monitor at the tethered station; even if it was a quick check this was a problem for the rhythm/connection. With the IQ I find that I can simply bring the camera a few inches away from my face and do a quick double tap on the area of interest and continue to shoot immediately, just fast enough that the model is still in the moment. Also, this can all happen before the image shows up in Capture One. The image is available for zooming just moments after capture on the back itself and it takes the usual 2-3 seconds to show up on the computer monitor.
Moreover the IQ's Focus Mask feature, and customizable highlight warning, works on those 10 most recent captures, and the default view shows the entire full frame (with nothing overlayed) and a small histogram on the side, which means you can check either the histogram or the unencumbered composition without any button pushes at all (on most cameras you either have to push a button to see the histogram or have to live with seeing the histogram always overlayed on top of the image).
The IQ also has a much nicer artificial-horizon / electronic level (2 axis) and embeds those values in the raw file, allowing Capture One to auto-correct (for a single image at a time, or many images at a time) the rotation and/or perspective of the raw after-capture. This won't matter much for shooting people usually, but comes in handy more often than I thought it would.
Finally the very long overdue USB-enabling firmware update will provide the IQ series tethering compatibility with a variety of smaller laptops that are not compatible with the Firewire only P65+ (note either back can connect using thunderbolt via adapter to firewire). It also promises very very good tethered speed (meaning final-shot-to-review-on-monitor time; the absolute frame rate is determined by the sensor/electronics not tethering speed).
Hitherto most digital back tethered shooters considered mission-control to be the computer and the digital back was just a feeder for that computer. With an IQ you can do so much more on the back itself you may find yourself going to the computer far less often. When you need to review in great detail nothing beats a 30" tethered monitor, but much of the time you're just looking for quick confirmation ("are the eyes sharp?") in which case the IQ screen is even better than the tethered monitor.
Shooting to CF card the obvious difference is the screen for checking focus etc but I don't think there is any difference in shooting speed with both being 1.0FPS. Of the two, the P65 has the bigger buffer (1.3GB vs 1GB) but possibly slower spec RAM? Real world will be unnoticeable.
I agree that shooting speed won't be meaningfully different. Technically the IQ supports the UDMA7 spec for CF cards which means if you were shooting 50 images in a row without any pause the IQ would finish writing the last image several seconds faster than the 65+. However, in practice its very hard to fill the buffer. For most CF card sizes you'll run out of room on the card before you run out of buffer depth.
i have a p65+. i have thought long and hard about the justification of flash screen and few extra features of the iq160, but just cant quite see the return for my photography business for the extra 10k price difference. I never hit buffer (and i shoot fast), so besides the screen, theres not much in it. the files are amazing, but the iso is low (as is with all medium format backs). i wouldn't shoot higher than 200iso.
Paul, when you've got some spare time you might want to look back at some of your ISO400 shots from the 65+ and run them through Capture One 7. Here is a comparison of an IQ160 at ISO3200 in sensor+ mode:
For your commercial work I doubt you'd be be happy with the max ISO (unless the creative intention called for a grainy image) but you may find you can go one stop higher in C1v7 than in any previous tests you made in C1v6.