I think, to those asking why Kane would chose United over City, I have the following thoughts (rose-tinted glasses well and truly on).
It is one thing to be the next in the relatively long line of City players who have won titles, being part of a continued 'conveyor-belt of success' but the notoriety / legacy you could leave with United, if you were the spearhead for the return to the top table, would be something else entirely. RvP is held in very high regard by united fans despite (IMO) only one good season (which was a great season, I'm not trying to diminish it) partly because it returned United to winning ways.
If Kane was to come to United, combined with others (as unlikely as it would be for United to sign anyone else if we made that outlay for him this summer) and bring the PL back to United as well as (hopefully) returning to challenging for the CL, he would go down in folk-lore and have much greater, longer-lasting legacy within the club.
That, for me, would be a rather strong pull - much more risky, but also far more rewarding.