I have both.
Breastforms I usually wear them with a pocketed bra.
The breastplate I have is full torso one, so you put it on like a t shirt.
I’ll be very honest, I regret buying the breastplate. I’ve only worn it a few handful of times and here are some of the drawbacks : the edge/lip of the plate, it very noticable. It is very hard to find a breastplate that matches your skin colour closely, so it sticks out like a sore thumb. This could be due to the breastplate that I bought, but the boobs do not look natural. If you look at a woman’s breasts, they do not stick straight out but it kind of encompasses the whole chest. Mine sticks straight out. It isn’t natural looking whatsoever.
This maybe secondary, but it is a challenge to take on/off, plus it can get toasty.
True, if cleavage is your top priority, then breastplates is a viable option.
Breastforms do not give you the cleavage you want, but it is way easier to take on/off, not as toasty, and if you spring for the decent ones (not the cheapo ones from ali/amazon), they can look quite natural. Ones I bought from ali are ok, but they are too small for my bra band size, so it doesn’t look quite natural (but way better than the breastplate ones).
Because my breastplate is full torso, it also inhibits the exquisite feeling of the clothing you choose to wear. One of the reasons I like breastforms is when I put on a slip or a satin blouse, you feel it against the skin and it feels heavenly. You do not get the same feeling with a breastplate.