1/3.91 is almost 1/4 inch.
1/4.85 is almost 1/5 inch.
When it comes to camcorders, the combination of larger lens diameter and larger imaging chip = better low light performance. The larger diameter lens allows more light in to the imaging chip. The larger imaging chip deal with what little light is allowed in by the lens. Good glass and good silicon yield are the important parts - and they are expensive. One *could* have a small lens and large imaging chip or large lens and small imaging chip - the trick is to find the balance between lens diameter (light allowed) and imaging chip size (light processed). Whether the imaging chip is square or rectangular will have little bearing on video quality.
The Sony HDR-PJ260V and Canon HF R300 are consumer grade entry/mid-level camcorders. The lens diameter is small (37mm) as is the single imaging chip (the spec is not listed as "filter diameter" on the Canon because there are no threads surrounding the lens to mount a filter). As camcorders increase in price, the lens diameter increases as does the imaging chip size (and the ability to behave better when recording under low-light conditions).
The imaging chip size (whether measured diagonally or otherwise), lens diameter, video compression used, lighting, use of a steadying device (not using the camcorder handheld) and other factors go into determining "video quality". The imaging sensor is only a part of the entire system that determine what you see - and whether it is "good" or not.