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I would suggest a lot more research on the topic. The inet has some details out there to look at. Most block websites will show buck options.
However either option will work. The advantage to the buck on either side of the foam means its slightly easier to attach the window, trim etc. However you have thermal bridging issues. I would recommend putting the buck between the foam, so ripping it to fit tightly. You can drive long screws through the buck and let the ends hang free into the wall cavity. Once the concrete is poured, it will flow around the screws and hold the bucks in place. Prior to the pour, you can screw through the foam to help hold the buck in place. However you must brace the bucks to themselves. Also remember to leave an opening in the lower buck to pour the concrete into the wall below the window.
Again, sounds like you need to do some more research on the topic. Call the block rep as well, they are typically a good resource too. The info is out there, use it to your advantage. You only have 1 chance at an ICF pour, mistakes are very costly.

We did a combination of both ideas. We did a treated plywood full width of the blocks with channels on the inside width of the blocks which held them in place and would have the concrete fill the inner channel. Remember to make an access port on the bottom of the buck to fill concrete better under the window.

You might want to ck out the ICF forum at the greenbuildingtalk.com.