Some people do not like 3/4 length, finding them uncomfortable.
I use a 3/4 and I have no problems with them. However, you do need to compensate for the missing length by using other padding. Obviously, if you need to carry extra padding to do this, then you'd be better off with a full length, but if you can use clothes (or other items that you'd be carrying anyway) then it makes good sense and saves a few grams.
I use my polarfleece vest to go beneath my feet while in bed, and I use my polar fleece top & trousers as a pillow. This way, the 3/4 mat goes from my shoulders to my knees. The main thing is that it needs to cover your hips and shoulders, where the real pressure points are, and you need to have enough other items to pad out your head and feet.
Works well for me.
(I use a very old Thermarest 3/4 length, but I can't comment on which types of mats are better, because I've not used anything else since the old hard thin closed cell foam mat which was horrible to
sleep lie awake all night on).
Wiki page on Sleeping Mats.