The PH Buenos Aires website was updated by the time I checked it out in February, but reservations are still not being taken online or offline. (Soft-opening and prior special invitations have not yet been sent out either to my knowledge.) So out of the three mentioned, only two are currently possible and the Hilton is the one I prefer since it's more modern and I can get to/from the airport a wee bit faster at all hours and I've been one of their more frequent guests since it first opened. Location-wise, the Marriott is more "central", but the Hilton and Marriot are a sub-$2 (sub-6 pesos) ride or a 20-minute leisurely walk away from each other.

I don't think there is a bad major international chain/brand hotel location-wise for sight-seeing in Buenos Aires since a lot of sight-seeing requires not walking (or a long walk).