I'd say definitely go for the highest available rrpm. The Nf or N2 turbine acts as a dam to the hot gasses, the slower it goes (for a given fuel flow) the more damming effect and the higher the T5 - same heat input, slower air flow rate. Although the torque would go up slightly at lower rrpm, the overall thrust is ~ torque x rrpm and is likely to be less at lower rrpm.
HC