If you are quite happy with MSBuild, then I would stick with MSBuild. This may be one of those cases where the tool you learn first is the one you will prefer. I started with NAnt and can’t quite get used to MSBuild. I’m sure they will both be around for quite some time.
There are some fundamental differences between the two, probably best highlighted by this conversation between some NAnt fans and a Microsoftie.
Interestingly, Jeremy Miller asked the exact opposite question on his blog last year.