I'd say the porting is better. I've gone a ways into using GToolkit, and it has some very nice things. I especially like the infinite nesting of editors, which is like transclusion on steroids, and the driller. And it's already language agnostic and very extendable.

But when it comes to actually extending it, that's where the sharp edges start to cut. I was able to do a fairly quick, enhanced Python support extension, but one of the issues is getting proper class equivalence, and the back-and-forth passage of data is a major performance suck. Also things will grind to a halt if you push and try to process too much data at once on the Smalltalk side. Maybe having a fairly beefy machine will help with that though.