I have a 400/4000 - And it has lasted me for the past 10 years.
The 400/4000 would be your best bet. They have a more powerful motor then the 200/300.
When buying a tool like this, think outside the box, not just the foam stuff, your looking to use it for.
You will find many uses for this tool, to fix, stuff around the house.
If you really only want it to carve foam, then go with the 300.
You still want something with a little balls,
You should also invest in a Flex Shaft, It's a lot lighter on the hand, then holding the entire weight of the Dremel.
You hold it like a Pen.