The V-twin has the "right now" on demand power but they wind out sooner than the 4's. The 4 cyclinder has great power but you have to have the RPM's up high. As a buddy of mine once said, "you have to ride them like you're pissed off." I've always rode twins. I bought a Yamaha FZ1 and the transition just didn't work for me. I can tell you that the power just kept coming, and coming, and coming. The thing that I absolutetly missed the most was the mechanical music of a V-twin. :boob Just today I was riding my buddy's 1974 Triumph 500 dirt bike. He has a 1/4 mile track at his farm. We all took turns riding until we fried the clutch. It reminded me of my first bike; 1972 Triumph 500 Daytona. It too has a four speed transmission with the shifter on the right. Everyone of us got on it and hit the brake on the left to put it into 1st gear :roll Even though everyone laughed their ass off when someone did it, it never failed. As soon as someone got their turn to ride, they would mash on the brake trying to put it into gear.
I love the sound and low end of a V-twin. I've been considering an RC51 or other since I'm 6'4 220lbs. I'd keep the 6 if possible. Maybe Aprillia.. but you will probably pick up about 100 pounds in weight on a twin in most cases. And as mentioned, top end. Most people that ride liter bikes are hooked on the top end like crack. And also those that ride twins are hooked on the torque like crack.. one of those things.
In the end I think its a matter of what kind of riding you intend to do, do most often, and enjoy.
i saw the difference between the power curve one day when i met up with a group of guys one night. one guy who i beleive rode a gixxer 750 would do his wheelies and they were always looked like he was havin trouble keepin them up, the front wheel was back and forth in the air, then he jumped on a buddies sv650s and did one and it was the smoothest wheelie i have ever seen, just came up and sat right there in the balance point.