Just on the want v IQ thing, I like this...
This is mainly about mirror v mirrorless but they talk about smaller systems and IQ too...
http://www.luminous-landscape.com/essays/cameras/the_mirrorless_revolution.shtml
This is why I like Zach Arias. Just talks sense, and doesn't give a s**t.
Been saying this for ages. APS-C? "full frame"? Both are teeny weeny toy formats, so who cares. So far as I'm concerned, the only reason to chose FF over a smaller format is if you need to squeeze that little extra out of big prints. However... I'll tell you something here. I've only been using a "full frame" DSLR camera for around 2 or 3 years. It was AFTER I stopped shooting for money that I decided to do so, as it was AFTER I stopped shooting for money (although I still do the odd job here and there) that I started shooting for MYSELF. Only THEN did I actually need to make big prints because I started to exhibit my work. When shooting for clients, I hardly ever shot FF. The fact is, fewer and fewer people care about quality these days anyway. Those that did demand quality, I used medium format digital, not a 35mm DSLR. My personal work doesn't demand, or require the use of hired or borrowed MF gear, and there's no budget to sustain that, so 35mm digital is a compromise I've arrived at.
Here's an interesting thing. I just loaded an archived folder back into my Lightroom catalogue. It's an archive of images taken professionally between 2006 and 2013... for money... for real clients. Some small local businesses like local Estate Agents, some large, like the Carsleberg Group, and Beyerdynamic... some for private clients ranging from people wanting a folio of their cat, to test shoots for transsexual models... basically... the entire gamut of stuff a great many professionals shoot... which is incidentally... anything that you're paid to do. I'm just a photographic prostitute, as most professionals are. You give me cash, I give you a picture.
I filtered by metadata.
By far the camera used most on professional jobs? A D7000. Basically I used what was appropriate. Studio? Medium format... location, DSLR. Simple as that. There are even jobs in there shot with a Canon EOS 350D. It was the lightest camera I owned, so occasionally... when weight was an issue... out it came. Most of this gear isn't mine either. I borrow, rent, and once... even stole (although technically it was borrowing without permission as I did return it
) A good photographer uses whatever is A) Appropriate, and failing that B) Whatever is to hand. That list is only the digital images too... doesn't include 6x7cm, 5x4in or 8x10in film.
The other notable thing is there are only 7 thousand images there. Professionals don't go around spraying the landscape at 8 frames a second unless they shoot sport, press or weddings/events/schools.. etc. In which case you'll probably be more concerned about speed and professional build than quality of images.
So.. when to switch to full-frame? Whenever the hell you want to, as it's no one's concern other than yours. Just be aware.... unless you do something uber-critical, you'll probably not notice much of a difference... and even then.... you're still using a teeny weeny toy format, so who gives a toss. Stop thinking of "full frame" as the pinnacle of what you can achieve. It's not - it's 35mm.
Good photographers create good images... not cameras.
However... to finish off: Stop trying to find a justification. It's your money, you want it.. buy it... be happy. However... your photography will NOT improve as a result. So long as you know that.