Depends where your priorities lie.
Both should perform similarly enough, with the Swiftech offering you ease of use and a cleaner build while the XSPC offers you far better flexibility and ease of upgrading.
With the Swiftech, the pump is built into the CPU block (DDC pump) and the reservoir is built into the rad. So you have two components to deal with, means its a lot cleaner lopking and easier to use.
The XSPC kit has a rad, CPU block and a pump screwed to a res. More parts, but if you ever want to replace one then you don't have to buy two new components (the pump is removable).
I personally would go with the XSPC kit, water-cooling becomes quite addictive and you will no doubt want that flexibility to put in new/different components without having buy something you already have (I made that mistake by getting an X20 750 pump, needed something better, but to replace it meant I needed a new reservoir as well, adding about $30 to the upgrade).