Is the 70d using basically the old 7d 18mp sensor? That one is not at all competitive with today's not APS sensors in terms of noise. Depending on the APS and FF sensor you compare, you may be lucky to get one stop.
Huh?? The 70D is Canon's modern sensor -- it's their newest sensor that we actually have testing and reviews for, and the 7Dii uses the same or similar sensor (but we don't yet have testing information for it).
Secondly, people too often misunderstand DXO testing when they only look quickly at the scores. Unfortunately, DXO scores are kind of based on the first inning of a baseball game, instead of the entire game. But when you actually look at the full DXO testing, you see that.. for example, the 70D sensor pretty evenly matches a camera like the Pentax K-3 at high ISO.
For example, at a manufacturer ISO of 3200 -- The Pentax K-3 has a measured Signal-Noise-Ratio of 25.9, and the Canon 70D produces SNR of 26.3 (so the Canon 70D has a teeny tiny bit less noise than the K-3 at ISO 3200). Meanwhile, in Dynamic range, at 3200 --The K-3 gives you 9.27 EVs, and the 70D gives you 9.03 EVs. So the Canon gives you slightly less noise, while the newest Pentax camera gives you just slightly more dynamic range.
Anyway, remember.. this thread is about Canon APS-C vs Canon full frame. Sounds like your advice is to abandon Canon and go to Nikon/Pentax/Sony APS-C.
You are not gaining more resolution that today's APS sensors unless you have more than 24 mp. You very well may not be getting more DR, either.
You are misunderstanding resolution -- It's not just megapixels. It's also lens quality, it's noise performance, all contributing to effective resolution. Put a really cheap horrible lens on a 36mp camera, you won't necessarily get any more useable resolution than a 12mp camera.
As ISO increases, you lose resolution as it is obscured by noise.
My statement was that I would take a FF ISO 6400 over an APS-C ISO 3200 ---
Again, sticking with Canon since it's the basis of this thread:
At ISO 3200, the Canon 70D gives you: SNR of 26.3. DR of 9.03, and color sensitivity of 15.8 bits.
At ISO 6400, the Canon 6D gives you: SNR of 27.3. DR of 9.62. And Color sensitivity of 16.4 bits.
So as I said, you get noticeably better IQ in full frame at ISO 6400, than in APS-C at ISO 3200.
Will is be mind-blowing differences?? No. Looking at a little 4X6 or Facebook posting, you probably would never notice the differences. But looking at the images full screen, or large print, those differences will be noticeable.
My point is that your advantages are more to do with a modern sensor than with FF.
So APS-C sensors are improving, but full frame sensors are standing still???
It's the laws of physics. Larger sensors can deliver better IQ than smaller sensors. Yes, APS-C sensors are improving. And today's APS-C sensors can match or surpass yesterday's full frame sensors. And today's full frame sensors continue to be superior to today's APS-C sensors. (And medium format even better, etc).
Imagine if C/N didn't treat their APS cameras as second-class... Nikon's flagship APS, the D300, is over seven years old (not counting the add-video D300s from five years ago), Canon took five years to update the 7D, and both are pretty skimpy on premium APS lenses. I suspect that if they weren't trying so hard to push people upmarket, the FF allure wouldn't be as strong for many people.
The allure of full frame is that it's better. That's the allure of cameras like the RX100 over other P&S cameras. Larger sensors are better.
That's not to say they are necessary --- You can certainly get incredible IQ out of APS-C cameras. And for many uses, you would never notice the differences.
And APS-C cameras have the advantages of being cheaper, with cheaper and slightly smaller lenses. They have the advantages of extra reach on long lenses.
Most people don't *need* full frame. Certainly, no amateur needs it. But then again, you can say that no amateur needs APS-C either.
These are all tools. dSLRs are luxuries for amateurs. Full frame cameras are an even higher level of luxury --- that can be used to produce even higher IQ.
I feel like some APS-C shooters try to deny the advantages of full frame, to rationalize staying with APS-C. The reality is, there is no need for rationalization. There is nothing wrong with APS-C. It even has some advantages. But it's silly to deny that full frame also has advantages.