Well I've built both, and loved them both. But between the two, my favorite is the slabside. It's the look of the original Cobra, and while the 427 is more muscular looking, I love the refined almost delicate look of the original 289 . Delicate until you step on the gas.
I built a 289 to hi-po specs and while it's not as powerful as the 427, It's managable horsepower, easily doing 0-60 4.5 sec times. Here are some comparison pics.
427
289
John O