This is a loaded question. You will get arguments for both. I personally prefer a full length. It is true that a 3/4 length quilt will save you a few oz. and be a bit less bulky than a full length. But then you will have to bring a small pad for your feet. Thus adding back the few oz. and the bulk. I have never found the need for a pad to sit on so I find it easier and less bulky to stuff a slightly larger UQ than to pack a small pad. I also find it more comfortable. Of course if your pack has no frame then the pad comes in handy.
In the end it is personal preference.