Seeing as I've never had either top I can't respond with an expert opinion but I have been looking for one for quite some time and they're pros and cons to both tops. The Smoothline, especially for 86-88 vert owners, is a much easier fit. It basically attaches at the same points the convertible top does and also weighs less than the GM top. They are pricey @ $1599 unpainted and the back window is smaller so people with a discriminating eye will immediately notice.
The GM top will "stiffen" the chassis whereas the Smoothline is almost purely cosmetic. Among Corvette enthusiasts, it is much more desirable than the Smoothline. It takes quite a bit more work to install and remove and especially for the early (86-88) verts as you have to either locate a retrofit kit, which is no longer readily available, or hope you can locate the pieces at a Vette slavage yard or you could make your own pieces if you are handy with tools. They can be had for under $1000 and almost always one is for sale on ebay.
I'd say if you have an 89 and up vert then the GM top makes the most sense. 86-88 Vettes are better off with the Smoothline. Just my 2 cents.