I prefer the full system. It gets rid of the CAT Converter which will reduce some of the heat and increase power and most of all it reduce weight. But you can always start off with the slip on and if you want to get the headers down the road you can.
I dont know how the slip on produce better power with that CAT installed. It amazes me how people believe that unless the bike is shipped different in terms of what type of CAT installed/Stock Up Maps than what they ship to California. I know we have tough emission laws here but come on a bike with a CAT produce more power than a bike without a CAT, 20MM larger headers for more air flow and reduced back pressure with the same silencers? Im sure super stock will be adding that CAT back on next year.