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.
* drains camera power, you might even forget to turn it off
Requires a separate power source.
I'm also a big fan of replaceable batteries, but my logger really logs 24h, so I never found that limiting. I also admit I rather like dedicated devices than the all in one stuff because specialized devices are usually better at what they do, and it minimizes risk because of damage/theft/powerloss ... of course that's really a personal preference.
* much(!) less precise / updates pos. much more seldom
There'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.
* not always in the best signal position (you can put a small tagger in your breast pocket, ...)
If anything, that's a worse location. Your body soaks up signals. The farther away from that bag of meat, the better.
I found it works nicely if you move around, it was just an example - and in any case it will be better than a camera stashed away in my bicycle bag under layers of protection.
* camera is larger than a logger: if you want to do travel logs you always have to carry your camera around
A GPS logger is one extra device. Chances are, you're going to have your camera with you anyway. And if you really just want a log of where you're going rather than where you took specific photos, chances are, your smartphone also has GPS and can do the logging without the need for carrying yet another device.
A smartphone is also a dedicated device, I don't have one :-p and my last non-smart phone's gps performance as very bad for example when exploring and crawling though former ussr barracks around Berlin. Plus a smartphone's gps also drains power from the non-replaceable battery as you argued above that's why I like to keep devices separate.
* no wayfinder/geocaching mode (you cannot set coordinates and let the logger guide you)
But at least the in-body GPS *could* be adapted to do those things with a firmware change. An external logger without a screen can't readily be modified to do that at all.
I already requested a gps geocaching mode for Magic Lantern, I hope they do it sometime. And my $50 gps logger *has* a screen, that's why I bought it, so I can enter coordinates and find my way.
* 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.
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.
My 24h gps logger is set for 5sec interval, I never move more than the gps' precision in 5sec, and if I would I could set it to 1sec with very little more power drain if the signal is strong. As I wrote above, I doubt the 6d gets an extra satellite fix right after every shutter release (or does it?). Even if so, the interpolation between 5sec logging intervals will be still more precise than the gps fuzz.
Also, you have to regularly synchronize the camera's clock to the GPS receiver or you're in for a world of hurt.
Your camera clock should be so precise so that's not often necessary...
