Chalk and cheese. Totally different depending on how and where you ride and what you actually want from a bike.

I back up what Sunil_FiestaRS1800 said above (apart from id say a 2 stroke will be around on par with a 4 stroke twice the engine size in my experience)

It also depends on what tracks and/or what terrain youll be riding on. Different bikes suit different tracks/terrain/riders.

As with any bike, its all down to how you use what you have to its maximum potential. A well riden 125 can beat a 250 round many tracks. for example.

Ive always been a die hard 2 stroke fan, and used to race in motocross 125 and 250 2 stroke, and know for a fact id be quicker than i would if i were on a 4 stroke 450 on track.

Having said that, ive recently bought a road legal WR450 4 stroke. Not using it competitvely this time round, but for green laning. There as reliable as you like if you keep on top of the oil changes and air filters, whether riden hard or not, so id say go for a 4 stroke if not racing.

Only way to tell what will suit you is to have a go on both though and see what you think.