I currently have a crop sensor. I would like to know how difficult it is to keep focus with a full frame camera. Is it really hard or about the same. I am looking to get the 6d for video and for low light s video situations. Thanks for your input.

For same image size and t/stop, the full frame sensor will have a shallower depth of field. That means it will be harder to follow focus.
There are a lot of variables here. For me the biggest three are:
-How much movement towards / away from camera are you trying to keep in focus (and how fast is the subject moving)?
-How long a lens (tight a shot) is required
-How much is the lens stopped down?
If you are shooting motion that is challenging with a Super35 sensor, it will be even more challenging with a full frame sensor. That's the geometric reality.
I'm starting to deal with it more myself, having just acquired an A7s for low light work (initial tests show acceptable grain up to an ISO of about 40,000 for documentary footage of audiences in dark clubs) but I'm trying not to go that high by using 1.4 lenses on the camera. So depth of field could be challenging!

Do a little test to check your follow focusing ability. Have a friend pace towards and away from the camera - say from 15' down to 3' -- back and forth a few times for about 30 seconds. Record it (make sure you're adjusting both framing and focus). Watch it played back on a BIG monitor. Now imagine having a bit LESS depth of field.

Now I am thinking it might not be such a good idea to fo full frame. I worked in the news biz for many years so I am used to pulling focus however my lens are rokinon cine lens with a t 1.5 stop. I like the flip screen of the 70d which I have but do not know if I would use the amazing focus ability of the 70d. What I want is a canon crop sensor with great iso capabilities.

The 70D does too much hunting and pecking, and is very noisy with on camera mics. I just bought a triple shoe adaptor that I figure I can turn sideways so one shoe is sticking forward, over the lens. I'm hoping if I mount my mic there, it won't pick up so much noise.

Otherwise, during, say, the ceremony, we tap the face on the screen, let it focus, then tap the focus OFF until needed again.

As for manual focusing a 5d Mark ii. It just takes practice. Here is a highlight video I shot from a wedding I DJ'ed, I'm probably shooting 50mm f/1.8 this whole time

Skip to the 2:00 mark where it gets really dark, and it's just my DJ lights.

Krissy and Ryan's wedding dance highlights, June 14th, 2014 on Vimeo

This serves as an example of what should have been a fourth variable -- how pin sharp does your intended audience require? Go through the posted video by randomly clicking to view still frames to see how sharp they actually are. While this is a pleasingly soft look and a nice feel for a wedding video, providing equivalently semi-soft images might not be deemed acceptable for dramatic or documentary shooting.

Jeff, I mentioned this before, and I finally pulled the trigger, so I'll be trying this out in a week, but,

someone made an app called "DSLR controller", which should let you use a tablet as a monitor via USB, same as EOS Utility would on your computer. Not all tablets are compatible, but their website has a nice list of what works, and if you need to change anything. DSLR Controller :: Devices (http://dslrcontroller.com/devices.php)

I just ordered a (used) 10.1" Acer Iconia 200 for $125 that is supposed to work with no mods, just a cable. I then have a shoe adaptor to mount a tablet ($8? $15?).

I'll let you know how the speed/frame rate is. At the least, it should help during the slower parts of the day, like the ceremony. I'm hoping it will be fast enough for the dancing, too.

Hi,

I switch from crop to full frame and from my experience comparing DOF -
"f2.8 on crop" equals approx. "f4.0 on full frame"

As for manual focusing a 5d Mark ii. It just takes practice. Here is a highlight video I shot from a wedding I DJ'ed, I'm probably shooting 50mm f/1.8 this whole time

Not your fault but I find the focus during the dance part a bit all over the place and very distracting, it constantly goes in focus, out again, in, moves focus to another person and so on, so I"m all the time guessing what you are actually trying to show, this is the main purpose of a very shallow dof, you guide your viewer to the part that is in focus because no-one looks to a out of focus part of the screen.

I watch a lot of movies and if there is a very shallow dof shot in there the part to whatever they want you to look at the focus is spot on, also if a person moves closer or further from the screen. I can spot a full frame camera, especially a 5DII at a dark reception from a mile away when I see the operator struggling to maintain focus. The 5D3 and especially the a7s allow for much higher ISO shooting allowing you to increase the dof which makes it much more manageable.

I often get the question how I manage to keep my focus on steadicam shots at candlelit venues at f2.0 and that's because I use m4/3 camera's, makes my life a lot easier. I would really like to have a full frame camera in my bag as well but only to use it's very narrow dof capabilities for creative controlled shots.