I have used both, no problems from either one of them. I will say the Dart is more of a user freindly block with the oil drains compaired to the World. Dart is very similar to factory blocks with the drains in the rear corners going to the pan, frount to the timing cover,on the center over the cam. The World only has the center holes over the cam and frount to the timing cover, NOTHING in the rear, a brain fart on there part for the wet sump guys imo.
Now the World has huge main webing, dowled caps and more material just about everywhere in the block. I currently run a World block with a 4 stage dry sump for oiling, blocked off the center holes in the galley with vent tubes and put a scavenge stage thru the rear of the intake and it works perfectly.
I like the World better for me, but there are some ideas for you to think about before buying either. Holp it helps ya.
Yea i have owned both and the World block looks a little more stout and like cmf406 said they offer 6 more bolt/stud holes on each side of the block but you have to use there cylinder heads to take advantage of them. But if it was me I wouldnt buy either I would buy the FRPP Boss 351 block, my engine builder/machine guy said its the nicest on he has ever seen and kurgan said he uses it in his renegade motor, and it is what i will be buying soon. Ford racing had problems with the boss 302 block because they shortened the cylinder length to much but did not do that with the boss 351 ad they are cheaper than dart and world.