First, they are not remotely the same. Each is completely different.
Second, having seen all of them last week with a 5 year old girl, I would say Fantasmic. Actually, there is no reason you can't see Fantasmic AND the fireworks if you don't mind watching the fireworks from the Fantasmic viewing area, as they start within 5-10 minutes of the first showing of Fantasmic ending.
We actually paid to have the Fantasmic Dessert package, and we paid to have the WOC dinner package. If that is in your budget I would highly recommend it as it does cut down on your waiting time.
Fantasmic if you have the Dessert package, you will have seats (one person will have to go early to get the seats assigned). If you don't do the package, you will need to camp out to get a good spot quite a while in advance of the show. Others on here can give you a better idea of time, but we noticed people staking out spots already a couple of hours before the show. From what we saw this year and last year, most people that stake out their spots remain seated on the ground during the show instead of standing, which can make Fantasmic a lot easier for the little ones to see than WOC. DD(5) is terrified of Fantasmic at WDW, but she absolutely loves it at DL and does not find it scary at all. If your DD likes Peter Pan, there is a great Peter Pan ship portion of the show that DD adores. That is a big reason why Fantasmic gets her vote over the others. The live princesses that are a part of the show helps too. After Fantasmic, don't move and you will see the fireworks from there. We were even able to see Tinkerbell and Dumbo flying through a break in the trees. Admittedly the fireworks are more beautiful when you are facing the castle and are able to see it with the fireworks over it, but it is still great from the other area too.
World of Color is amazing; however, if you do not want to have to hold your five year old up to see the whole show, it may not be the best choice for you. While Fantasmic has real characters in the show, WOC merely has projections on the water. I say "merely", but let me say that although it might not sound impressive (it didn't to me until I saw it), it was incredible. The problem is that everyone stands for the whole show, which makes it next to impossible for the little ones to see. Last week, it seemed like there were people standing in line for hours to try to get a good spot. With the dinner package, we were able to go less than an hour before the show and get a good spot. We went around 8:20 or 8:30 which is late by the standards on this board. We were the 2nd row back from the rail above the wet zone, with DD actually having a spot on the rail (plenty of room for her without the people in front of us having to move an inch and they invited her between them, just not room for all 3 of us). The wet zone rail already had a few rows of people. Even though DD was on the rail, she was not tall enough to have a completely unobstructed view of the show between the rail itself and the group of people below that filled in the whole wet zone by the time the show started. I would have thought she would be able to see over them, but she was not happy she could not see the very bottom and that her face was rail height, so she begged DH to hold her to see better. He wound up having to hold her up the entire 20-25 minute show, and she is not light! I'm short and if we had not had a good spot near the rail, I would not have been able to see the show either.