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I am looking at prefinished floor because of the 25 years warranty.
You should know that the warranty actually covers very little. The flooring needs to basically self destruct before the manufacturer will honor a claim. Warranties do NOT cover scratching, dents, cupping, shrinkage, abuse of any kind (as determined by the manufacturer), etc. So do not purchase a floor for the warranty only.
Brazilian teak is a durable wood and either the 5/16" or 3/4" should be fine. However, the 5/16 " floor will only be able to be resanded one time, whereas the 3/4" floor can take multiple sandings. You should really consult with an experienced flooring installer/contractor prior to purchasing your flooring to ensure you do not have installation issues that can limit your choices. And as to cost, some stores do charge more than others. But there is also a difference in quality as well. It pays to get educated about wood flooring.