The first step to deciding this is realizing that there is no "better overall" option here. There is only a better option to the situation you have as far as actual growing space is concerned. It is true that a 600w bulb does product more lumens per watt than any other HID wattage. This efficiency does in fact allow you to run 2 lights vs 1 1kw light for just around 200 watts more and maybe a tad less depending on the ballasts being used.
With that said, now the factor of space and even coverage does come into play. I have a 6x6' area lined with orca film and a 1kw hortilux hps in the middle with a reflector that provides a good pattern for moderate spread and penetration over the area. I did at first try 2 600w for the reason your mates had suggested but I ran into a problem. With my area size, I had a region in the middle where each of the lights spread intersected. The result was uneven growth; being wild in some and lacking in others. The 600w had spread the light very well but would not penetrate like the 1kw. After replacing with the 1kw, I got the right combination for my area by having a very even spread with better penetrating power.
Some insight from someone who went through something similar

I think JUST FROM MY EXPERIENCE 600w would be well suited to equal 3x3 spaces in a rectangular formation or 4 in a 6x6 which has 4 3x3 areas. Just some insight.
-PE