Concerning gps: I disagree, and here are my reasons, but as always I'm open to being convinced otherwise :-)
But first off: I've got nothing against built-in gps if it's thrown in there for free, but as it stands it's likely it's used as a marketing item and adds the same amount or more $$$ then an external logger.
If it does, it's because they're choosing to sell it that way. The actual cost is orders of magnitude less, because 95% of the cost of an external logger comes for free when put in-body—no battery, no case, no additional CPU. All you have is a GPS chip that costs maybe a buck or two, an antenna that costs maybe a buck or two, and possibly a slightly larger logic board to hold the extra chip, for a grand total cost of maybe $3–5.
* much(!) less precise / updates pos. much more seldomThere's absolutely no reason why this should be the case. In fact, if anything, the reverse should be true. The camera should query the GPS for its actual position when you take a shot
In theory, yes. But do you think the 6d takes the signal on every shot?
Hardly, like other embedded gps devices it's build for power saving rather than precision, in real life the logging frequency of a dedicated logger in the trunk can be so high you can get a precise log of a fast moving car.
From a power perspective, I'd expect a track log to be worse on the average because the GPS has to be brought back up to an active state regularly even when you aren't shooting photos. The cost to ask for another reading is basically zero while the chip is up and running. But that probably depends on the workload.
In an ideal world, a device like this would switch into track log mode if you shoot a picture and it has no signal, and should then apply the first position to any photos taken while the signal was unavailable. This would still require less power than continuous track logging, but would eliminate the most obvious issue with taking a measurement when you click the shutter.
* no a-gps (camera needs longer to find the satellites).aGPS can cut this so-called "warm fix" down to a couple of seconds by fetching that data over the cellular network.
Nope, on better non-smartphone loggers with a-gps you can also upload this data from the pc for the next months or so thus severely shortening the fix time when it's cloudy.
With the caveat that if they have to make an emergency position correction on a satellite to avoid hitting a piece of space junk, your computed location could be off by hundreds of miles. There's a reason they only guarantee the ephemeris to be accurate for 4 hours.
Thus, unless you're very rarely taking pictures, the warm fix time affects you once per day, and only if you don't power up the camera until just before you take your first shot of the day. After that initial warm fix, every acquisition should be a "hot" fix, which means that it should take about as long as it takes you to move your thumb from the power switch and pull the camera up to your eye, give or take a second.
Amen. I didn't try the 6d gps and I hope it works this way... but at least my last phone traded precision for power saving, but maybe today's gps chips are more advanced.
The one disadvantage of the gps logger that you have to attach the track log to your pictures afterwards, but this is done with one click in Lightroom or other apps. For me, in-camera gps is a gadget I could really do without.The bigger disadvantage is that it has no idea when you actually took a picture, so you have to choose a query interval that trades battery life for accuracy or vice versa.
Cell phones have much smaller batteries than your camera. The iPhone 5, for example, has a battery capacity that's less than a third of a 6D's battery, and that's with not just Wi-Fi and GPS, but also a cellular radio. :-)
Your camera clock should be so precise so that's not often necessary...
Digital clocks drift, on average, by something like five seconds per day. In two weeks' shooting, that's an error of more than a minute. Maybe the clock in recent Canon cameras is more precise than that, but I wouldn't hold my breath.