"Purchasing" a tranche usually means taking a long position in the bond (which is equivalent to selling protection). "Selling" a tranche means taking a short position (buying protection). A decrease in default correlation reduces the likelihood of many defaults occurring together, which benefits the senior tranche (making it safer) but increases the likelihood of at least one or a few defaults, which hurts the equity tranche. An increase in default probability hurts all tranches, but the equity tranche is particularly sensitive to it. Therefore, the equity tranche's value will decline significantly due to both higher default probability and lower default correlation, making it attractive to sell. The senior tranche benefits from the lower default correlation (offsetting some or all of the increased default probability effect), making it relatively attractive to purchase. Thus, purchasing the senior tranche and selling the equity tranche is the best recommendation.