Vortech will be the cheapest option, you can pick up a new Vortech kit for around £3K from Assaultech in the states. It will make you the most power but its pretty noisy.
HKS kit will cost a fortune & won't reach the power of the Vortech, however it is a better built kit, sounds better than the Vortech & is much more refined. Its also a bit more torquey than the Vortech.
ATI not seen many people run these here or even in the states & that says a lot imo about the kit.
Stillen sounds amazing, doesn't make a lot of power & you have to have the gash bonnet fitted with it. You'd probably make more power with boltons & uprev than you will with the basic Stillen kits.
Personally theres only 2 I would ever go for - HKS & Vortech.