Estimation is the process of finding an estimate. It is not very commonly used, so I would go with "estimate". I think the last time I saw the word 'estimation' was in elementary school math. "Use estimation to find the answer."

'Estimation' is also used mean opinion or regard. For example, you would say 'After I heard about his disgraceful behaviour last week, he really went down in my estimation'. Again, this is quite old-fashioned. 'Estimate' is the more usual word. An estimate is a rough calculation. For example, before a contractor does a job for you, you would ask him to work out an estimate of what it is likely to cost.