By far, VFP is with not discussion a better tool for desktop apps (except than Delphi 8 .NET, the only tool capable for build cross 32bits/.NET app that also are fast), also .NET is so diferent to VFP that that meaning a full rework (except the DB). But .NET is the future and is very clear that only .NET can be consider by MS "first citizen" by next release.

So, if your product requiere a large amount of work, then move .NET can have a little sense, but is more a in-house solution or deployed limited, then leave it in VFP.

But in the end, you must take in account simply facts: .NET is slow in desktop, next release of .NET for desktop in next OS is ANOTHER beast (= rewrite actual winform, if want leverage the plataform, because winform is "deprecated" because AVALON) and this is not the bad thing(because a a kind of upgrade path exist): Next os requiere a huge upgrade of machines. So take in consideration the kind of clients that you have and learn more about the next big .net things...