@Mortein:
Now that we know that Kaido really wants to catch Momo and extract information from him, which he doesn't have, which means Kaido is going to kill him, I don't see where else he could go other than in SH's crew. Minks can't protect him, they were not able to defeat Jack, and his fake father can't protect him either. Realistically only Law/Luffy alliance has a chance, and given how his father was a part of Rogers crew, it's seems appropriate that he become a part of Luffy's crew.
@Sparsebeard:
So you believe he's gonna be ready to become Shogun just like that, a ten years old Shogun? It pretty much ridiculous if it happens, we must not forget Shogun can be translated as something like generalissimo, having a boy become so would be a disaster waiting to happen. That's my opinion anyways… Staying on the crew would make sense as it would allow for some tempering.
Momo already has a potential job (cabinboy), a dream, a tragic backstory, close relations with Luffy and the crew, quirks, his personnality doesn't really overlap the others (unique enough anyways) and he already followed the crew for a while. Plus, he wouldn't take much needed fights from others (it's already hard enough to have everybody fight in an ark without it seeming forced).
In my mind, the writing is on the wall...
Also, being an orphan of some kind seems to be a plus for crewmates candidates.
How would it be "safer" with the Straw Hats than in Wano? The reason Wano's dangerous right now in the first place is because Kaido's there, and he's allied with the shogun of the nation. When the StrawHearts go to Wano to take the samurai home, they're not just gonna say, "Okay, it's been fun, bye!"; they'll fight Kaido and the shogun since they already have to fight Kaido anyway. Once Kaido and the shogun are defeated, Wano will be free, and there won't be a problem with the samurai staying there. Momo doesn't have to become shogun; he'd become daimyo of his region (which is a step down from shogun) like his father before him, and he'll have Kin'emon, Kanjuro, and Raizo–all of whom he seems to be quite close to, and who should have plenty of knowledge after serving Oden for so long--to help and guide him with that.
Being with the Straw Hats, on the other hand? An infamous pirate group who consistently get into fights with super strong guys, are very wanted criminals constantly being hunted by the Marines, and seek out the most dangerous adventures possible (thanks to Luffy)? Unless you're a strong fighter, being with them is not safe at all. Even "cabin boys" Buggy and Shanks were old enough to fight and hold their own when they were with Roger, and there were a lot more people on Roger's crew. When you've got a crew as small as the Straw Hats, who have that many enemies and are that far along in the New World, they all need to be able to look out for themselves. Momo is an eight-year-old boy who hasn't fully and properly learned how to fight, who can't control his Devil Fruit transformations, and who can't really do much in his dragon form yet anyway. He would be far more of a burden to them than a help. The only writing I see is that he'll be very similar to Vivi: someone whose country is in trouble, who will travel with the Straw Hats and have other adventures with them on the way to that country (which has been happening already), and who will stay there to rebuild said country after the Straw Hats save it.