Mary,

When you say stability what, exactly, do you mean? It is resistance to tipping over sideways? If yes, then a wider stance in better, i.e. a cat vs a mono.

Or do you mean how much the trailer "dances" around behind the tow vehicle? If yes, then a longer distance from the trailer hitch to the axel will have a much greater affect than a wider stance. That distance being longer will cause the trailer to follow your tow vehicle better (it will center it self better) and it will make backing up much easier.