So, I've heard arguments both ways, like since they are targeting diff demographics that the availability of the other will not effect the initial sales of the launch week. The other is, even if there demographics might not align, they are still competing for that one purchase that people will make that week. It will force people to choose which one they want. There are other arguments too, but what do you think? Here's another question just for the hell of it. Will the wonderful 101 flop even harder in the U.S. because of gta. v then other territories?(like it's possible for it to open lower then it already did in japan xD)

Think about it...you have to consider gamers who:
1.) Own both the WiiU and a Ps3 or a 360.
2.) Are interested to buy both games
3.) Don't have enough money or time to play both.
I think we are talking about 50 copies at best.

Thats true, wonderful 101 is much to niche anyways so prob not the best comparison. Hypothetically, what if the witcher 3 and destiny release on the same day?

All consumable media, to some degree, is in competition. The average person only has a finite amount of both time and money, which leaves them with decisions to make. After that, it's a matter of breaking down the demographics and seeing how much overlap exists.

Of those Wii U owners who also own a PS3/360, how many are interested in GTA5? Quite a few, I bet. Another factor, I think, is that those who will buy GTA5 are likely anticipating playing that game for a long time, and thus are probably not thinking about other titles. Because of that, I think GTA5 hurts everything around it.

Still, it's going to be mostly the Nintendo Faithful who buy a niche title like W101. GTA5 will affect sales but, because W101 isn't going to move a ton of units in the first place, the actual impact won't be extreme.

Every game competes with each other, I think.

Yes in that case there would be more competition...it would be difficult to quantify. Still I think you should consider 2 things: how much demographics are distant to each other...for example jrpg and fps are quite distant demographics I guess, while open-world action games (like GTA or Red Dead Redempion) are are closer to fps games.

2nd You should consider how much choice does one audience have: for instance, if you like GTA, which is quite unique in terms of scale and quality for its genre, than you are likely going to buy it anyway...on the other hand if you like fps games you have a lot of choice, every year you have a better COD to play (or Battlefield or some other game) so, unless you're an hardcore fan, you can "afford" to skip one entry of COD in order to buy GTA.