IMHO, I'd go with the auto, unless you want to constantly churn through the gears. The auto shifts faster, and esp with a tune, I don't see any reason WHY you'd want to clutch/shift all the time. Right now I've got a borrowed automobile with a 5-sp manual thrashbox & manual clutch... I'm already sick of shifting. It's just no fun anymore. This actually was a revelation to me, because I was leaning towards the 6-sp manual. Having to shift all the time sucks. FAST thrills me, but Old-school, stick and clutch pedal Forced Shifting (whether for fun or grocery-getting)doesn't... not anymore. Of course, I'm not a kid, either. With the auto, I can still 'manual shift' if I want to.
Now if Caddy offered F1 shifting (as does Maserati or Ferrari) with REAL shift paddles behind the steering wheel (and no clutch pedal), I'd gladly reconsider.
But hey, you might be totally different. Either way I think you're gonna have fun.