Good to see you back Kevin! Hope all is well.

In adition to Kevins comments, different server settings and available resources. That said, If you're getting a spill to tempdb in one but not the other, the usual suspects are massive desparities in row counrs (already mentioned by Keni) and/or out of date stastics on one instance but not the other. Also, check to make sure both versions are set to use the same compatibility level and the same version of the cardinality estimator.