I'm using 5DII alongside a pair of 7Ds and previously used it along with several 50Ds.
No, I don't think FF AF is any more or less accurate than crop. At least if there is any difference in their accuracy, it's not enough that you'd ever be able to detect it. Maybe you could measure some difference with some kind of scientific tests... I dunno. But I don't think it's enough that you'd ever be able to definitively detect it by eye and point to it.
I suspect more likely that any differences are more related to the user. Besides the greater magnifications you tend to use when viewing crop camera files that Lloyd mentions above, when using the same lens on both formats, assuming you are framing your subject the same way, you will be standing considerably closer with the FF camera. That makes your effective depth of field shallower and more visible in the larger viewfinder, so you can better evaluate focus accuracy and make fine adjustments or refocus before taking the shot. Also, most folks use a FF camera for more sedate types of shooting (landscapes, cityscapes, portraits vs sports/action), which are a lot less challenging to focus accurately. Because of this difference in subjects, we're also more like to use One Shot with the FF camera, AI Servo with the cropper. One Shot tends to be more accurate than AI Servo.
And, no, the focus systems of 30D/5D Classic and 50D/5DII are not the same. They appear similar because you see 9 points in the viewfinder, and the center AF point on all of them perform pretty similarly. But the 5D series have 6 additional "hidden" expansion points right around the center point (can be enabled or disabled, only work in AI Servo though) for an actual total of 15 AF points. The 30/40/50D do not have those expansion points at all.
Also, the 30D and both 5D models have only one cross type AF point, at the center. 40D, 50D, 60D and 7D have all 9 points cross type, though the center one is still more sensitive. 5D outer point performance drops off pretty dramatically, probably because it shares rather heavy image file data handling through the same processor that's performing AF functions. The 7D has dual processors for image data handling, plus a dedicated processor for AF (similar to 1D series cameras). The 30/40/50D all also share duties through a single processor, but files sizes are smaller than with the FF camera (Note: 50D's files are larger than the 5D Classic's, but it's processor is two generations newer, too).
I don't know that I'd call 7D AF "better".... It's purpose and design is different from 5DII's AF. One camera and it's AF system are designed for action, the other for more sedate types of shooting. Certainly the 7D locks on faster (but interestingly the tests comparing it with 50D showed the older cameras slightly faster locking on... mere milliseconds difference though, not enough you'd ever notice in use). And the 7D handles AI Servo tracking of moving subjects better, as might be expected with a more action oriented camera. I don't think the 7D is any more or less accurate, though. And the 5D models will keep focusing in about a stop's worth lower light, albeit pretty slowly, after all the croppers' AF systems have given up completely.
Finally, 7D, 50D and 5DII have Micro Focus Adjust feature (does 40D also have it?). Used right that can improve focus accuracy quite a bit.
