I'm considering buying a 1.4tc to go on my 400mm 5.6. I've often wondered, which is better in terms of IQ:
Using 400mm (no tc) on 60d?
Using 400mm on 5d3 and cropping?
Using 400mm on 5d3 with a 1.4tc?
These options all yield a similar focal length, but which would give a better overall result?
When using autofocus to capture a quick image, the second two methods might yield better results, for a simple reason, AFMA. You cannot fine tune your 60D autofocus to give you the best autofocus accuracy (liveview will help, and so will manual focus if mounted on a tripod and used with liveview). You can tune your autofocus with a 5D MK III or 70D too, for that matter. On a 70D, live AF will let you add that 1.4X or even 2X TC. Being able to use fast enough shutter speeds to prevent motion blurring is also often a advantage in FF.
Being able to fine tune AF or use live AF on a 70D might very well improve the images merely due to focus accuracy.
You might also wait for a couple of weeks and see what features will be in a new 7D replacement. It may have better live AF that is capable of tracking and even faster response and focus accuracy. There is a possibility that this will be the best for inexpensive use with long lenses, at least until the next generation of FF bodies, and if its done right, the only advantage for FF bodies will be shutter speed, since the large sensor gathers more light, you can use higher ISO's and faster shutter speeds.