I'm not familiar with the brand Perlas de Mallorca, but I own various Majorica brand strands.
The shell pearls I've seen were nowhere near as good looking as vintage Majorica (brand) imitation pearls. Some of the current Majorica pearls I've seen had a pseudo-orient that seemed too prominent to be believable. However, not everyone takes care of their imitation pearls and the surface can be damaged by contact with perfume etc. So vintage eBay Majorica strands do vary in luster. As always, buyer beware.
Shell pearls are going to be lighter weight than glass-bead imitation pearls-- in the very large sizes this could make a difference in wearing comfort.
I have to think there is probably a range of quality in shell pearls, but from the ones I've seen, they are significantly less expensive than Majorica brand. A plus if you are watching your wallet.