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As much as I respect GNUStep, I think Enlightenment is a better choice if we truly want a Linux DE on par with Aqua. Unless, of course, you're referring to easy cross-compiling of apps. In that case, yes GNUStep would be a logical choice. But I honestly think E17 is closer to a "production ready" state, as well as being more modern and (dare I say it) prettier.
Also, GNUStep is more than just a DE, it's an entirely different way of approaching desktop computing than the normal Linux model. You can put Windowmaker on any Linux distro and it's just a look-and-feel thing; spin up the GNUStep live CD and you get a better idea of how great the whole GNUStep philosophy is.
How about this: Finish both, put them on top of Debian Squeeze and see which one comes out on ahead. I'd root for either one myself.
Don't forget about EtoileOS. Now with Debian/k-freebsd I would like an Etoile/GNUstep desktop for this variant.
The libraries and underpinnings of E17 are supposedly already used for mobile.
But the question becomes, why would Nokia with their QT use it ?