Optimum depends on the system in question, there are too many variables to say which is better without either testing both or a detailed analysis of the actual system. I'd say get the larger - you can always throttle it back.
Good thing you warned me, I would have got the Maxijet 750 otherwise. There's only a $3 difference between those pumps, so price doesn't mater in this case.
Now I need to find out the outer dimensions, so I can know if it will fit within a 5cm wide resevoir or not.
200gph will probably be fine. i had a 150 in my system for a while and it did ok it will give you a decent temp you could get a lower temp at a higher gph with the right setup personaly ive got a 500gph in mine