I think the difference i them is probably the same difference as the ones they sell in la senza, but im not sure the websites a little hard to see the difference in the two. In la senza one types is designed to go under the breast and sort of lifts up the breast, incresing cleavage, and the other which is bigger sits in the front of the bra and basically just fills the bra out more. I have the latter and they look like the silicone enhancers.
if ur not wanting to spend a lot of money, matalan do something similar for about �6....much cheaper, my mate bought some but i could only see ones for cup size A and B so if ur any bigger there not much good, there a bit fuller than the ones in m&s and la senza so it depend on the dress and bra ur wearing aswell, im sure u dont want them showing. There a great bargin for the price, however the ones from La Senza whichwere about the same price as the ones in m&s area bit better quality than the matalan ones, so probably the m&s ones are about the same quality, but if ur only going to use them a few times what does it matter.