http://www.growlightexpress.com/pages/ceramic-metal-halide-pv-c0-2.html This is a retailers website. I have found multiple sites that are offering 250W CMH and 400W CMH. Just some information for ya. Also no i am not affiliated with them in anyway. Google just happens to be my best friend.
And my last post to you sir is to check out this link. With a little research i found a website with some basic FAQs on lighting. Yes it is a retailers site and it might be skewed as to sell you the lights but none the less information is information right...
You might also want to reference this thread as this particular grower used a 150W CMH and had some really great results for his first grow. https://www.rollitup.org/grow-journals/430541-150w-cmh-scrog.html
I know this link has been put up in other threads but i had a difficult time finding it. http://advancedtechlighting.com/cmhfact3.htm Watch the first video and you should get a pretty good understanding of the differences. It will all still boil down to personal preference in the end. It might...