The 6.7 will not be as easy or cheap to mod in my opinion...I just finished a ISB, ISC and ISL course...The 6.7 has a water cooled VG turbo that act's as an exhaust brake as well so you won't be able to add a standard exhaust brake...It has a common rail fuel system, unlike the 5.9 which is not as easily changed and the worst thing to hurt Performance, the EGR system with a DPF....The more fuel you add, the faster it will plug and the more back pressure created.....It's a strong engine right out of the box with huge potential but it will be costly to tweek it.