If one was in Wa St, either the 450 or 350 would have a licensed wieght of at least 14, if not 16K lbs gvwr. As OUR DMV takes tare wt times 1.5, to the next higher ton, and that is what you are licensed at, and LEGAL too!
Reality is, take the truck in this case, with the tighter turning radius, heaver springs, axels etc, license it for what you need, and start enjoying the rig.
I've had multiple rigs, licensed OVER the manufacture gvwr, even the axle wt ratings, been pulled over, wieghed, NEVER have I had an overweight ticket! as MY PAID for license was over what I weighed in at. Only once when I weighed just over 27K with my navistar, licensed at 26K, did I run into problems, and even at that, no issues IMHO. I was given a 10 day pay for a 28K tab, and sent on my way! as I was under the 19200 graw and 9600 front axel weights they enforce too.
Either of the two trucks will get the full max 20K on rear axel, and probably 9500-12K on the front. So technically, here any way, they could go down the road at 30K gvw and still be legal. I would prefer the 450 personally at this wt than the 350!
GCWR wise, that is not a legal term per ANY SP in ANY of the 50 states, so that is also not a factor. It is more do you have enough payload to handle the load per your limiting factors!
Speed wise, both rigs will pull hills about the same, same max grade in first gear starting at, assuming the same motor HP/torque specs, axel and trans gearings, tire diam etc. Braking will be less on the 350 vs 450 for the truck load itself.
Marty