The disadvantage of V/S and LVL CHG is that any altitude constraints in the route will not be protected. This is particularly important for someone new to type or unfamiliar with company routes etc. An autopilot coupled to VNAV PATH or VNAV SPD will assure compliance. If you are descending in VNAV path and you want to fly a constant ROD for a while, switch to VNAV SPD and over-ride the autothrottles from flight idle into ARM mode. You can then adjust thrust to achieve the required ROD. This is similar to using LVL CHG, except that when a waypoint is sequenced that has an altitude constraint; LVL CHG will bust straight through it. VNAV SPD will revert to VNAV PATH and comply with the restriction.