We've had 4 or 5 200's in the world, and just recently replaced two of them with 3300s.
There used to be huge differences between the two. The 200 is/was a key-system, and the 3300 if a PBX. The 200 was what you wanted for a hotel; it had all the hotel PMS integration you'd want. The 3300 was meant for large companies. Over the years, these distinctions have faded, and now the 3300 can do almost everything a 200 could. Still a few features missing, true on-touch day/night button that lights when in nights.
Originally the 200 was the only system to have "Call Park" obviously that has been brought to the 3300 now.
Like Chakara said, the 200 has a much better interface for transferring calls directly to a voicemail box. It has always been a "hack" to do it on the 3300.
DID handling, the 3300 has always had the better options.