How to choose between UC and PL when using the DPLL algorithm?
If you use the original specification of the DPLL algorithm, in which the unit rule is applied to a fixed point and then the pure literal rule, then only the unit rule is needed to reach a satisfying assignment.  If the rule order is reversed then the pure literal rule is used to eliminate the final unsatisfied clause.

DPLL is a complete and sound algorithm for deciding Boolean satisfiability even if the pure literal rule is removed.  In practice, pure literals are typically removed only during the preprocessing of a SAT instance, where the formula is simplified as much as possible before beginning recursive search.  This is because detecting pure literals during search is computationally expensive enough to more than offset any expected runtime gains.