I suggest test driving them both if you can. If the financial circumstances permit, the drive should convince you. I've driven both but own a 4.6is...there is little power difference, noticeable at higher speeds. The 4.8is has some upgrades over the 4.6is and feels a bit more refined. The 4.6is is less tamed, more like the wild stallion of the bunch.

Whichever you decide, you won't regret