Cost is a big factor in this. St Tropez will be a lot more expensive than Marbella. It's a question of glamour destination vs classic tourist destination. Both have beaches, both have sunny weather. St Tropez is more cultured.
If cost is not an issue then I'd choose St Tropez. If you just want a relatively cheap and cheerful beach holiday with a load of other tourists then choose Marbella.
I should add that I love Marbella and the surrounding areas, I've been several times. I've also been all around the south of France. I'm giving you the truth as I see it.