Fat Sam is totally correct. If you have the money buying like three 600 watt lights is better than two 1,000 watt lights. But I'll be a little more specific for you. Let's compare the 600 watt and 1,000 watt HPS bulbs, and I'll stick with Sunmaster because it's a good light and different brand bulbs put out different lumens.
600 watt super hps = 90,000 lumens and 215 PAR watts
1,000 watt super hps = 150,000 lumens and 358 PAR watts
Now if you go with three 600 watt lights, that will be 270,000 lumens and 645 PAR watts
Now if you go with two 1,000 watt lights, that will be 300,000 lumens and 716 PAR watts
Initially it looks like the two 1,000 watt lights have the three 600 watt lights beat by 30,000 lumens and 71 PAR watts. But that's not the whole story, as you are using 200 more watts (2,000 vs 1,800). The 600 watt efficiency is 90,000/600 = 150 lumens/watt, so if you had 200 more watts worth of a 600 watt light, that would be 30,000. That would equal 300,000 lumens per 2,000 watts of 600 watt bulb light or 150,000 lumens per 1,000 watts of 600 watt light. This is the exact same efficiency as the 1,000 watt light, so as far as Sunmaster bulbs go, the efficiency of a 600 watt bulb and 1,000 watt bulb is the exact same. This goes against everything that I've ever read.
Now as far as PAR watts go...
215/600 = 0.3583
358/1,000 = 0.358
So you do get 0.0003 more PAR watts per watt with the 600 watt bulb, however, this is obviously next to nothing.
I guess the fact that you can place the 600 watt lights closer to the plants would make them more efficient. But then again, if you have air cooled lights and are blowing a lot of air through them you can still get the 1,000 watt lights pretty close. If you decide to run either two 1,000 watt lights or three 600 watt lights the main advantage of the 600 watt lights is that you get better light distribution.
Also, tckfui said something about the electronic ballasts being more expensive, which isn't true. The price is pretty much the same these days. At least it is over at htgsupply.com and a couple other stores that I've seen. I think a good ballast to get would be the new electronic ballast that allows you to change power from 25-50-75-100%...with 100% being 600 watts. Although, I'm not sure how high quality it is, but it looks pretty nice. Here is the link
http://stores.shop.ebay.com/MGS-Supply__W0QQ_armrsZ1
It is the orange ballast and I just noticed they now have a 1,000 watt adjustable ballast too, but it costs 142$ more.