I never used WANT, but NANT works fine with any Delphi version. I presume NANT is also better supported and has more features you can use since it is used more widely then WANT.
BTW: You can still use MSBuild and call dcc32 if you want. In that way it is the same as NANT. If you move to Delphi 2007 MSBuild has a bigger advantage over NANT since the Delphi projects are in the MSBuild format. But even then you can still use NANT as a buildscript. It will just call out to MSBuild to do the separate builds.
Another build tool that is worth checking out is FinalBuilder. It is very complete and knows Delphi. It should, since it is build using Delphi. They also offer a CruiseControl like Continuous Integration solution. The drawback is that FinalBuilder is not free.