most side-folders lock and function a little more solidly than under-folders.
Thats been my experience as well. Ill also add "a lot" to the solidly part. Most all the under folders Ive shot/handled, had wiggle to them.
The "issue" side folders, also give the proper LOP and cheek weld, and dont interfere with the controls when folded. They also open quickly, with less effort/fiddling. Once open, they shoulder and shoot like their fixed stocked counterparts. Ive always found the under folders dont shoulder as well.
Not all side folders are created equal either. The couple of aftermarket stocks I tried were lacking. Stick to the "real thing".
One advantage to the wire type side folders is, they can replace the standard stocks on some AK's, simply by removing a couple of screws.