Welcome to the site !!
Like everyone else mentioned , 250 or 450 depends on a lot of factors , weight , skill , riding preferences .... As Ray mentioned , a 450 in the wrong hands can be dangerous , or at the very least , beat you to death , you will be struggling to hold on if you get too aggressive with them , whether a older one like my 02 or a 2020 (a 2020 450 will really wear you out , 60 HP is nothing to take lightly , for example a stock 02-08 450 has about 55 HP in stock form , trust me , 5HP difference is a LOT !)
Your weight as a factor , a 250 is set up suspension wise for a 130-150 lbs rider , a 450 is set up for a 150-180 lbs rider , it will beat up a lighter rider , and a 250 with stock suspension will beat up a heavy rider
Skill as a factor ..... if you are truly a beginner track rider , a 450 is going to get you into trouble fast , and the outcome won't be pretty , a hospital trip may be a huge factor ...... a 250 whether you are skilled or a novice can get you into trouble , but it is much more easy to wrangle around a track .... jumps can bite you no matter that bike you are on , and no matter how skilled or not you are .... I speak from experience
Riding preference .... a 250 is going to be much easier to trail ride with , especially if you are not heavily experienced , a 450 again , can get you into trouble before you know you are in trouble .....
If you are a experienced trail and track rider and fit the weight category ... or have the funds to respring the bike for your weight (springs for weight , valving for skill) either a 250 or 450 will suit you fine .... you will get less tired on a 250 no matter what type of riding you will be doing , they weight about 20-30 lbs less than a 450 and are just easier to throw around , and at the end of the day , you will be much less tired on a 250 ..... The 450 will have power everywhere , sometimes where you don't want it .... you will spend more time in the first 1/2 of the throttle on a 450 than you ever will on the last 1/2 ..... a 250 you can gust give it more throttle till it runs out ........ how I compare 250/450 power comparisons is , a 250 is like riding a 450 in 3rd or 4th gear that is about the output feeling , a 450 is like riding a bull , but instead of just 8 seconds , your doing it for 15-30 minutes
More detail of what your skill , plans , and ability is would help a lot , your weight is also a factor , also , is trail going to be the major riding for a while ?? , or short term then moving to track ?? , or back and forth between the 2 and both right away ??? , if so how much actual track time and jumping experience do you have ?? (be honest , saying you are a B rider when you are a novice will get you hauled away on a stretcher real fast , your skill is your skill , it is nothing to be ashamed of , or over boast your skill about) , all this factors into what bike you would be more happy with ...........
If you can ride someone's bike (250 or 450) this will either tell you what you will be happy with , or scare you into riding the smaller of the 2
EFI , is much less temperamental when changing elevations , or after picking the bike up after a fall and trying to start it , but both my bikes have carbs and I can ride anywhere my friends with EFI bikes can .. it just takes a bit of tweaking on the knobs to do so ...