Depends on the horse. I've used both, I like the soft, but some horses take to the hard, so suppose you just have to try em and see.
Also you have to take into account the thickness. Some of the vulcanite 'hard' bits are quite thick, which wouldn't suit a small mouthed pony or horse, the soft rubber ones seem to be slightly thinner. Have you looked at the happy mouth?? same sort of thing, they do a pelham.
AND then if you have a bit chewer, they damage the vulcanite, and make them rough.
Cheap rubber bits, are awful keep away from these. Only buy a recognised make. A few being sold on ebay are cheap muck.