The 300CX is far superior to the 300X, no doubt. When I bought my 3120 I went looking for a 3320 or 3520 with 300CX, but they had a 3120 demo with the 300X that was FAR less than anything else with the same warranty. The price was just way to good to pass up, but I always wished I'd went for the extra with at least a CX.
Using both the JDQA and SSQA, I can say that each have their benefits.
The full 'loop' on the JDQA is A LOT easier to hook up attachments vs the little piece of sheet medal on the SSQA. You can be on uneven ground and it just encloses the pin nicely. On the SSQA, there is much less forgiveness, so they are harder to get aligned.
The SSQA is a lot easier to latch (two levers) than the JDQA which is a couple of pins. I can latch the SSQA 4 times faster and without having to go to each side of the loader.
For the smaller tractors, I'd much prefer the JDQA as it is much lighter and a lot less too it. Though for tractors like my 110, the ability to go rent some attachments is very nice.