How and where you plan on flying will determine your power needs. The further you fly and the more obstructions in the area the more power you'll need to maintain a safe link.
I fly my 250 with a 200mw transmitter - but I fly in the desert with no obstructions and never much more than 500m away (battery won't get me much further and back again reliably.)
General rule of thumb on RF is to use as little power as necessary to get the job done (technically that's the law...you don't say where you are but in the US either 200mw or 600mw will require an amateur radio license and once you have one you should be able to answer this question for yourself.)
With the extra speed you can go farther, but the big issue is if you loose signal things can go wrong faster at speed. If you're flying through trees - you'll need the extra power for sure. If you're flying over an open field and above obstructions between your antenna and your multi - 200mw is fine. Trees block a lot of signal at 5.8ghz so it takes a lot more punch to get through them and even if you're flying slower and can handle a short signal dropout you'll probably need 600mw to get more than a few hundred feet away.