Many people assume that agnosticism is more reasonable than atheism because one can't prove there is no god. This reasoning is based on two false assumptions. First, that you can't decide that something doesn't exist if you can't prove conclusively that it doesn't exist--which sneakily includes the false assumption that the atheist has to prove god doesn't exist. Second, that all propositions are equal.
The first proposition is demonstrably false. Just look at the way we all treat other propositions, such as unicorns, fairies, Santa Claus, Zeus, and Thor, just to name a few. We have very little doubt that none of those things exist. Yet we can't prove they don't--at least not conclusively. So, how do we know they don't exist? Because there is no proof that they do. It isn't up to us to prove that god doesn't exist. The advocate of the proposition is the one who has to prove it. If there is no proof of a thing, the logical conclusion is that it doesn't exist, and you should simply say that.
"You know as well as I do that if there is no proof of a thing, the logical conclusion is that it doesn't exist. You apply this reasoning yourself all the time: to fairies, Santa Claus, Leprechauns, etc."
Also with regard to the first point, acknowledging that you could be wrong is not the same thing as being unable to make up your mind. This is something I would like to say to all agnostics. Atheism isn't a belief, it is a lack of belief. Most atheists do not claim absolute certainty. In fact, I have met very few who refuse to admit that they could be wrong. But, if you let a seemingly remote possibility of error prevent you from making a decision, then you would never be able to make a decision about anything.
The possibility of being wrong should be sufficiently large to give one pause before one reserves judgment. If the possibility appears to be very small, then it should be nothing more than a reminder to be open minded to new evidence, should such ever appear.
Admitting the possibility that there could be a higher power doesn't necessarily make you agnostic. The question revolves around the likelihood that you assign to that probability, and the likelihood that a proposition is true varies with its nature.
Do you think the existence of god is about as likely to be true as the existence of fairies? If so, then you are probably an atheist. Do you think there is about a 50% chance (or some other significant probability)? Then you are probably agnostic.
You do not have to be 100% sure of god's nonexistence to be atheist. You need only lack a belief in god and be of the opinion that god's existence seems quite unlikely. There are different levels or types of atheism, those that claim to be a 100% sure are called "strong" or "positive" atheists. Some who consider themselves to be "strong" atheists will admit to an infinitesimal amount of uncertainty, but many positive or "strong" atheists, and all or almost all agnostics, think that one does have to be 100% certain.
The second point affects the amount of proof required. Not all propositions are equal. Like the question of assigning a probability, the amount of proof required should increase when the proposition is sufficiently extraordinary. Just as a great deal of proof would be required to prove the existence of unicorns or flying saucers, a great deal of proof would be required to prove the existence of the a supernatural being with unlimited (or nearly unlimited) magical powers. The odds of god's existence are not 50-50. The extraordinary nature of the proposition puts it in the same category of probability as unicorns, at best.
If you call yourself agnostic, but think that the possibility of god's existence is on a par with the probability that unicorns exist, then you are more properly classified as an atheist. You have misclassified yourself because you have mistakenly and implicitly bought into the false notion that the burden of proof is on the atheist. I discuss why this isn't true in other posts.