the WR will also have a different ratio gear box to the motocrosser so that does tame it down a bit in the lower gears, will also have a higher top speed. of you wanna compare it to the other 450 you rode (crf450) weight will be immediately noticable. you have things like starter motor, battery, lights, oil catch can, radiator overflow, kick stand, extra baffels in muffler.
it doesnt matter if your a average rider, above average rider, superawesome sing my ****s motocrosser, if your a smart rider, you should be able to handle the 450. it will only ever go as fast as you twist the throttle. on the flip side, get the 250, then when you are comfortable with that, get a power pipe, then ignition, then some weight saving parts. then upgrade to the 450.