I can understand getting bent out of shape over the whole 2 stroke/4 stroke debate. There are advantages and disadvantges to both designs but a lot of the opinions you find on the internet have been posted by a 12 year old who probably doesn't even ride. I've never seen an objective cost of ownership comparison starting at day one between say a yz 250 and yz 450f that were ridden the same and maintained per factory reccomendations. I will do this myself as soon as I win Powerball I:I
My opinion based on experience (which is not that of a professional motorcycle mechanic, just a dude from WA that likes to go fast and work on bikes) is that the cost of ownership on the 450 will be ~20% higher over the life of the bike if you keep it long enough to get in to engine rebuilds. This 20% comes from the cost of premium crank case oil and the additional parts in the top end. This cost can be kept down substantially on both designs if you figure out how to work on your bike and do it yourself. I also think that 4 strokes will go longer between rebuilds as long as you take care of them.
The reason I warn people to do their homework before buying a 4t and to steer clear of 250f mx bikes is because of the experience I had with my first real race bike:
I was 22 and got a "good deal" on a YZ250F; it was an old race bike with an awesome aftermarket suspension, full ti Yoshimura exhaust, and a Vortex ignition with mapping switch that would let it rev to 14k. It didn't run quite right when I bought it, but coming of an XR200 I figured (incorrectly) that it just needed a plug and an oil change. I took it to the shop and had the guy sweating to get it tuned correctly, it cost $500 for a valve adjustment and carb rebuild (Kehin FCR carbs are awesome but also complex). After this it ran like a bat out of hell but was still a clapped out p.o.s. I loved riding it until i took it down to the dunes. I was pinning everywhere and flying off of everything, I should have been taking more breaks to keep the motor cool but I was having a blast. Eventually the tired bike had enough and threw the rod through the bottom end. The cost of a new cases, rod, cylinder, piston, valve job (from being clapped out not grenading) etc. was in the neighborhood of $1800 to do myself
I ended up parting the bike and lost my ass on it.
TL;DR: If you are going to buy a four stroke race machine get the nicest one you can find and bring a friend who knows how they are supposed to run. If something seems off, it is probably going to be expensive to fix
Here is a two stroke article for your time:
MOTOCROSS ACTION'S TWO-STROKE VERSUS FOUR-STROKE SHOOTOUT: 2004 CRF450 VERSUS 2001 CR500 | News | Motocross Action Magazine
I don't care if I'm not good enough to win races on a 500AF, I want one