Just ask yourself will the $3k matter to you at the end of the year, and if $100/month more is comfortable....then OF COURSE get the 5.0i. If you are second guessing yourself now, you'll be kicking yourself if you get the 3.5i. I think the 3.5i is GREAT, and plenty power for me...but the 5.0i is in a league of it's own!!
Get the V-8! Just be prepared to be spending A LOT more time at Chevron with the 5.0i!!!!