This isn't an either/or situation. The ReactOS developers collaborate heavily with the WINE developers, so many updates to ReactOS also benefit WINE and vice-versa.
I just don't understand. WINE is like a necessary evil, a noble sacrifice of its developers' time to enable users to switch from Windows to a free, open platform for the applications they can't run on one natively. It's a sensible abstraction that enables something that wouldn't otherwise be possible.
But why reimplement Windows, the whole OS? If the devs enjoy it, it's their time to use as they choose. But the end result would be much more beneficial for users if it were put toward WINE so users could leave the Windows platform behind.