Splatoon will sell well but so will Halo 5. Both great FPS games. What's your point of the comparison? Are you trying to claim 1 game being better then the other? Both exclusive for there own consoles, both different types of games and completely different atmospheres.
Halo 5 is selling well digitally and remember Halo 5 is also competing with giant games like COD, TR, Destiny, FO4 and many other big releases on X1 while Splatoon doesn't.
Sales dont tell the true story here, I personally think Halo 5 will have longer legs then Splatoon and will probably end around the 5m mark which is still a very successful game for MS and Xbox while Splatoon will probably die out around 3m. Plus Halo 5 has probably already sold more then Splatoon since there are no digital sales included here. Just my judgement and opinion here. I am personally a fan of both games but Halo is Halo.