Dude, try searching in future...
Decat will give more outright gains...what it will do to your overall curve i'm not sure. Sportscat simply means you never need to worry about swapping come MOT and might make you feel more environmentally concious...
You don't need to weld in a decat, there are a few off the shelf options for you from Janspeed, Yozzasport, K-tec Racing etc.