Depends a lot on what you are after in your career. World specializes in something that is only a small portion of ATA's portfolio of business.
I really love cruising the globe in a very cool widebody, doing exotic flying and laying over in interesting places. If this is your bag, ATA has a strong but diminishing niche within the company that does it, and your likelihood of getting to do it as a new hire is low (by the ratio of new-hires going into the L-1011 I guess about 5-10%).
World, on the other hand, ONLY does exotic worldwide flying in a cool widebody. It is all they do, and they are growing their fleet and their book of business. If you go there, you are certain to see things and go places you have never thought of. The crews I have met are really cool too.
ATA is my choice for lots of reasons, but the biggies are this:
-In direct comparisons of the widebody charter flying, the ATA contract has excellent pay provisions and time - off protections. From what I understand from the World crews I talk with, they get far fewer hard days off per month. Though I think the base rates of hourly pay at World are pretty decent, the ATA crews that want to can make huge bonuses for selling your days off back to the company (being JA'd a few days into your days off can bump your paycheck anywhere from 30%to100% depending on the trip). SO overall I gather it's a lot easier to bank a lot of extra money here. (We need a World pilot to fill us in on this - I do not know the facts)
-ATA has a wide variety of flying, and when I get sick of jet-lag, montezuma's revenge, and no TV in english when I can't sleep in the middle of the night in some faraway country, I can go fly scheduled service domestically in new airplanes under what I think is a very good contract.
-I smell ATA wanting to do scheduled international service in the future, that will mitigate the wind down of the L-1011 program for me.
On the last point, the L-1011 and the heavy military charter business ATA does is not a certain part of the future. The L-10 will be on property a few more years (only the 500's a a couple 100's are still left), but there are real questions whether or not a widebody charter program will continue past that point. The next widebody on property, if I had to guess, will likely be set up more for scheduled service than charter.
So, in sum, if you are yearning for the adventure of worlwide widebody flying to places other people can only dream about, World is a good way to ensure you do that.
At ATA, you will have an opportunity to do it, but will have a growing established "normal" airline to work for when the adventure wears off.
Good luck......if you have a choice between the two you are a fortunate person.