Debit: Funds are taken from your wallet on the site offering the card. No need to fund it manually. Once you pay for something, BTC is exchanged instantly at the time of payment, not before or after (you won't lose money due to the volatile exchange rate).
Prepaid: Funds must be loaded by the user manually. Funds are converted once loaded so you may lose money due to the volatile exchange rate.
But with a debit version you have to keep your coins on a third party site and we all know that's a bad idea especially long term. If the site gets hacked or disappears then you can say buy to all your coins too.
That was a major issue a year or so ago, less now with the rise of licensed exchanges like https://Bit-X.com.
I don't think being licensed makes them immune from thefts or scamming users themselves (you can always have rogue employees too). Where is the exchange licensed?