The "better ones" recommended are those made and supplied by BetterScanning
, which typically do a better job of holding the negative flat, to help keep the image in focus for the flatbed scanner. I have one for my 120 medium format scanning with the 4990 scanner. It is excellent.
That said, I still use the standard neg holder pictured above for scanning my 35mm negs. It seems robust enough for sensible use. I've probably scanned about 200 rolls with it in the last few years. One of the BetterScanning holders, especially with ANR glass, would probably do a better job, but the standard neg holder is adequate for my needs.