From a feature perspective there are newer features in R10.6.0 that are not available in R10.5.1 but R10.5.1 may have some fixes that are not in R10.6.0. (Given that it is a more recent release).
My advice, glance at the release notes for both, if you do not need any of the new features in R10.6.0 (Over R10.5), then go with R10.5.1.
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