Estimation in Agile is a relative estimation process. It's a guess, a place to start from. This can be a difficult concept for management to grasp, where facts are preferred so they can hold us to account. This comes from a lack of understanding of the development process and how the full understanding of the work does not emerge until the actual work is done.
Hence we give our best estimation based upon what we know at the time. A story point estimate assigns a value point to each item. The value itself is less important than the relative value, because a 2 for one team has a different meaning for another team.
Where this becomes a real problem is when the team equates points to hours. Once they do that the points are no longer relative. This is where using t-shirt sizes instead of the points can be useful, until of course the team begin equating them to hours.
T-shirt sizes are a little simpler than story points and there are less arguments between team members than when using points. We all know the concept of T-shirt sizes so the team acclimatise quickly to using this estimating technique. The size range is less than in poker planning. In T-shirt sizes the usual range employed is XS, S, M, L, XL and XXL, 6 sizes in all. In the fibonacci values in poker planning, generally we use 0, ½, 1, 2, 3, 5, 8, 13, 20, 40, 100 and "?" unsure. This larger range can take longer to estimate, but is still very valuable, especially for an experienced team.
In conclusion, I would recommend using T-shirt sizes for new scrum teams, and in your retrospectives evaluate the value and accuracy of the estimations. Then at some future point, try poker planning and compare the results.