I had an 08 KTM 450, then went to a 2010 KTM 300 and now to a 2012 570 Berg.
The simple facts are that:
In every area the 4 stroke gets more traction.
The 4 stroke wieght and inertia helps you 'plow' through rock gardens and rough stuff where the 2 banger will deflect a fair bit.
The 4 banger is easier to ride harder for longer and is less tiring.
The 4 bangers are easier to ride if you lack some experience. I could ride my 300 at 99% pace of my 450 over all terrain and probably 105% on really tight singles but it is harder to do so and requires more rider inputs and control.
Engine braking helps most poeple during cornering as they are more forgiving with general braking.
Awesome on tar roads and long fire trail sections.
FI - no need to mess with jetting or A/S adjustments.
The 2 bangers are a ball 2 ride, the engines on the new Bergs/KTM's are so versatile and relaibale it's not funny.
They do feel much lighter to pick up, especially when compared to the 4 stroke Bergs with their high crank mass.
They are much more nimble and flighty and can change direction really quickly.
The new 300's have heaps of torque when set-up properly.
They are more tiring to ride fast and hold onto when on the pipe, but the sound and feeling will make you smile from ear to ear.
Mixing and carrying oil IS annoying.
They will never tractor like a 4 banger through the slop, a good rider may mask the weakness in this area with his skill, but, like for like, when it gets slippery it's 4 banger territory.
Very easy to work on but are susceptible to jetting and air screw changes to keep them at their best.
They suck to ride on the road and long fire trail sections.
Either are great bikes, it's just depends on your skill level, what you want out of a bike and the sort of terrain you normally ride. As long as you make an informed decision you'll be sweet, there's nothing like a test ride though.