But as simple says it is hypothetical and the difference between 40% of 1000 and 60% of 600 is p*ss all...
Since it ultimately boils down to cal in vs cal out, the difference, as far as weight loss is concerned, is really 100% of 1000 vs 100% of 600, which is quite a lot.
The bottom line is, the more intense your workout is, the faster you burn calories. So that means that more intensity is better -- so why not just go all out ? The reason is that you'll quickly get overtrained/injured if you go 100% every workout.
So if you want to be smart and strategic about it, the right way is to find a middle ground where you can get in a reasonable amount of volume without injuring yourself, and maintain a good enough intensity that you burn calories at a good rate. In fact a better way to think about it is this -- calories is ultimately proportional to how many miles you run, so just go at whatever pace enables you to get in a good amount of mileage without too much discomfort -- probably a fairly slow pace.
Another point worth mentioning -- running is inherently a fairly intense activity, so even "easy" running burns calories much faster than say spinning on the recumbant in front of the TV (as often seen in your local gym) Moreover, you need to run at a fairly easy pace if you want to build a decent tally of miles for your training week.