for most folks slip-ons are good enough... they drop some weight (up to 12lbs in some cases), sound good, give a few more inches for the corners, and might open another horsepower or two. unless you're planning engine mods slip ons should be fine. full systems usually are larger in diameter and the good full systems will significantly increase ground clearance. the larger diameter won't usually do much on a stock engine but would definitely help out for larger displacement mods later on. spaghetti headers will scavenge exhaust better while providing the best corner clearances. i believe the old fbf forza's and some of the old dp headers used to just be an oversized stock header... they didn't really give you that much without engine mods and provided no additional corner clearance. my understanding that the current fbf stuff is just rebadged sil moto and those are the better spaghetti type header much like the current termi's. i replaced my stock header with a staintune spaghetti system after a laydown... the staintune spaghetti was only about $100 more than a oem header and cans back then (now they're more like unobtanium)... but i'm slowly but surely changing out stuff on the engine.