Thanks for all the replies, comments, suggestions, experience reports, etc. I'm going to respond only just where I have to add something that might add to the discussion. Still, I really appreciate all of them and I'm happy that there were also some voices against the 6D/FF now.
milkrocks said:
This comment is from someone who's only had a 6D for about 18 hours. Compared to my previous XTi the noise performance is incredible. I'm not a pro nor am i interested in performing super detailed testing, but playing around last night with a friend's 2 year old has me really excited.
ISO 6400 looks better than ISO 800 in my XTi (maybe even similar to ISO 400). The 24-105 F4L is marvelous. I spent a lot of time making this choice and am very pleased with the result. I plan to post an amatuer review comparing the difference between these two systems.
I really don't need high ISO much - or at all. And I wouldn't call f/4 marvelous but at least it's a stable value over the whole zoom range so it's decent.
Marsu42 said:
Based on that: stay with your 550d

at least it runs Magic Lantern (timed bulb exposures, unlimted hdr bracketing, focus stacking for macro).
The 70d sensor isn't a significant upgrade, and you don't seem to have the need for a better af system. A full frame isn't really better in all cases, as it has a more shallow dof and this is what you *don't* want for macro, and for landscape it depends on how much you want to boost the shots in postprocessing.
I really got rid of the 550D already (before I expatriated, recently). Right, I've got to give ML a try once in a while. Agreed, that FF is not what would be ideal for macro but then again that's not my priority but just a nice thing I do now and then. And I really like to keep my postprocessing down.
gigabellone said:
Marsu42 got a serious point. The switch to FF is going to cost a heap of money. You need a serious wide angle lens for landscapes, even more than serious if you're going to take pictures of stars. To save some money, you can do double-duty with a 100/2.8L for both macros and portraits. That said, i would get a 6D body, an EF 24-70/2.8 II and 100/2.8 macro, for a total of about 4200€, 4500 if you throw in a decent tripod, but i assume you already have one. Of course you can spend a lot less, but what's the point of getting a brand new shiny full frame camera and skimping on lenses?
Serious wide angle lens for landscapes? Didn't feel any necessity there yet. And no, no stars. Yes, I do own the 100/2.8L Macro lens and yes, I did use it for portrait in the past. I also have a 50mm/1.4 which is better for portraits in most cases. Yes, the EF 24-70/2.8L II sure must be nice. But also quite expensive as you say. I basically know I've got to get it if I go FF but not sure that will be right now. Any recommendations for a standard zoom lens that is in the <$1000 price range?
Marsu42 said:
So what? What does the op need high iso for - posed portraits? no. (tripod) macro? no. architecture? nope, not with vanilla lenses. landscape? only for the superior postprocessing leverage of ff - and in this case, a used 5d2 might be even or better (a bit more mp, a bit sharper at base iso).
The other advantage of the ff is thinner dof, looking at the op's gallery he could profit from that - but expensive zooms or primes are needed for it ... much more than just a camera body upgrade. Last not least the infamous 5d2/6d af plus missing crop factor isn't made for shooting squirrels or alligators from a distance, so what he ends up is our popular setup: either a 5d3 or 70d/7d+6d combination. That's a lot of $$$ for camera bodies that loose value in no time, it might be smarter to get some nice primes first.
Actually, high iso might be useful in macro (think of things that might fly away, etc). But I still try to avoid it and never went higher then ISO 800 - and that is already unusual. So both options should be fine I guess.
Thinner DoF sure is a argument for the FF. Again, except when shooting macros. Figure I really need to either get a cam for macro alone (well, maybe plus some lighter travel photography) or stop doing it at all. If I go FF, that is and you guys are convincing in that point.
Oh, Squirrels and Alligators. (US) Gray Squirrels do get really close. Actually, in NYC or Boston, you have to pay attention they don't come bite you in the finger if you're eating a sandwich on a bench in a park. So getting close is really easy here. And Alligators. Well, it's not like that one had been free to go anywhere further away. Unfortunately (for him) :/
Nice primes first, maybe second (APS-C) body later. Understood.
Sella174 said:
Unless I missed it somewhere, the OP never states what EF lenses are in the equation ... and there's lots of iffy EF lenses that actually perform quite decently on APS-C, but sucks on an astronomical level on FF.
There's actually only a few lenses. They are:
EF-S 17-55mm/2.8 IS USM
EF 50mm/1.4 USM
EF 100mm/2.8L Macro IS USM
EF 70-200mm/4L IS USM
iron-t said:
If you're anything like me you're going to miss the EF-S 17-55mm f/2.8 if you move up to FF. I loved that lens on my 60D. The 24-105mm f/4L that I replaced it with upon moving to 5D3, while serviceable, is far from lovable, and it'll be awhile before I can afford a good FF f/2.8(L II) zoom to replace it.
Which brings me to my point: if cost is a factor, don't be too quick to jump to the 6D. Not only will you need a new normal zoom, but the high quality wide angle zooms available for FF are much costlier as well. Consider this: while I adore my 5D3, I'm thinking down the line of also picking up a 70D and EF-S 10-22mm (which if you've never used one is superb). At that point it'd be hard not to think about getting Sigma's 18-35mm f/1.8 just because.
Just thought I'd give my counterpoint to all the full frame fanatics out there. There's no question that full frame does provide certain benefits. For landscapes and macro, you can stop down more before diffraction starts to rear its head. You have a little more dynamic range overall and much less noise at high ISO. You can get shallower depth of field for portraits. But there's also no question you can capture great images with a 70D. Indeed, take a stunning shot on APS-C and make a big print out of it, then challenge any gear snob to tell you how big the sensor was.
Thanks for the warning and tips. Looking through the lenses on the market it really is as you say - I'd miss the 17-55mm/2.8 if I can't afford the 24-70mm/2.8L II right away. Get's one thinking.