Yes VJW is right about this detail. Off course, VNAV SPD will maintain its VNAV features and will fly exactly all the altitudes restrictions that you have set in the FMC (that's why it is VNAV).
What I meant is in case there is no altitude restriction (most of the time in my part of the world), both of them will fly you with idle power until you reach the MCP altitude...but it goes without saying abouth the VNAV features.