Your maths is only correct if you end up keeping the phone exactly 24 months and you have to just hope the network doesn't increase your tariff. If you keep it longer, giff gaff is cheaper, if you keep it less time, you can sell the handset while it's still worth more. The handset is also not network locked or "branded" so worth more.
I'm sure you can stick it on a 0% credit card, but it's up to you. Personally I don't like being tied in to a two year variable rate contract.