That depends on your goals and how you drive the truck.
A non-wastegated turbo will give you decent boost at high RPM's with low exhaust temperature, but at the price of lag and very little output at low engine RPM's (i.e. when you're starting up).
A wastegated turbo will give you better boost at low RPM's and should give you similar boost at high RPM's (unless the wastegate's set to open at too low of a boost), but at the price of a smaller exhaust turbine, which will give you higher exhaust temperatures at high engine RPM's since the exhaust won't flow as freely.
If you do a lot of in-town driving, you'd be better off with a wastgated turbo; if you're mostly on the freeway and aren't all that concerned about acceleration, a non-gated turbo would suit you better. With that said, since F-450's are geared so much lower, different factors may come into play...hopefully an F-450 guy can give you a better idea if what will be best in your application.
As for price, a new turbo system will be considerably expensive...expect to spend $2K+, and IIRC the only systems you can buy new are the Hypermax non-gated setup and the ATS Aurora wastegated system. Used turbos can be found reasonably easily enough...wastegated turbos are generally more expensive than non-gated turbos.
Hope this helps a bit...good luck!