Next Gens are equipped as standard with two FMS and VNAV. Having two FMS is way more convenient than a single installation, and PSA has LPV approach approval - which is only available in the VNAV aircraft. LPV approaches are way better than LNAV!

There are about a dozen old -700s with single FMS/no VNAV and about 25 other -700s with dual FMS/no VNAV. The rest of the -700s, and all the -900s are Next Gens with both.