Simple, the first is less assuming. There's nothing wrong with either. If you were to say (See, I did it there) if you say, then most native speakers will assume that you're leading it up with a negative consequence, or something that will happen to you, simply because of how it's commonly used, here in Britain at least. E.g "If you say a bad word, we will punish you." The second is just a more tutor-like or friendly way of saying it.

I don't often use the first conditional because it is too presumptive. - If A happens, then B will happen. The implication here is that B will ALWAYS happen when A happens. Life is not this predictable, so I prefer more "wiggle room" in my statements.

There is, as Dylan mentions, a tendency in English (especially British) to be as indirect as possible (within reason). This is because it is usually a more polite, less aggressive way to talk. In this case, it is worth noting that the first conditional is designed for conditions which are real and tangible, not hypothetical. A couple of examples: If you don't study, you will fail the exam. If you sign this contract, we will loan you the money. In those cases, the use of the first conditional assumes that the condition is going to be met. This means that in the first case you're almost accusing the listener of not studying. And in the second case, you're leading your listener to sign the contract, you're assuming that he is going to sign.of course, in some cases, this is exactly what you want, but in general it's a "pushy" way to speak. The second conditional is for hypothetical conditions, where they're not certain/real. So: if you didn't study, you would fail the exam. If you signed the contract, we would loan you the money. In both cases there is no assumption conveyed that the listener is failing to study, or expected to sign the contract. So, for that reason, it is more detached, less forceful, it allows the listener to decide for himself whether the condition is acceptable/applicable to them or not, and therefore, it is more polite.

Both sound correct to me - it's really a matter of personal style and context/situation. Your first example sounds more polite to my ears. The first conditional certainly seems more direct, even though it could refer to the present or future.