I made the jump to a 450 from a 350 a few years ago and I love it. If you go with the 450, just order the higher gear ratio. The factory offered 4.88 and 4.30 when I ordered mine in 2002. I went with the 4.30 and have not had any problems with it. My unloaded mileage averages right around 16-18mpg, but it drops to 11-13mpg when it is pulling the boat or the big car trailer. My top speed with no engine mods is 95. This is without a load, of course! If you need to go faster than that, install aftermarket gears or buy a sports car!
Boot is right that the acceleration is not as quick as the 350, but the increase in braking power more than makes up for it. Any truck can get a load rolling, but stopping it is the big issue. That's why I stepped up to the 450. Without a doubt, the 450 stops a load much better than my 350 did. The 450 has the advantage here simply because of the larger brakes.
Also, if you go with a 450, be sure to get the aluminum wheels. This will save on rolling weight over the steel wheels. Something else to keep in mind about the larger wheels. The factory 19.5" wheels on the 450 may weigh a little more than the factory 16" wheels on the 350, but they also help offset the lower gear ratio because of a slightly larger tire diameter. The 450 with the 4.30 rear end does wind out a little more than a 350 with a 4.11 rear, but it is not really that big of a difference.
Overall, I would buy another 450 in a minute. This truck is an absolute work horse! If you want the best of both worlds, get the 450 with the larger brakes and then call Boot for some engines upgrades to make it go!